Ayup, really, a high quality full range PA speaker, of the appropriate size, and properly set up; will always be more accurate, and often sound better, than all but the best guitar or bass cabinets; while being lighter, more durable, and MUCH cheaper.
What do you mean by "High Quality and Full Range" ?
By high quality, and full range, I mean a speaker that:
1. Is sturdily constructed, with no rattles, undamped harmonics etc
2. Has proper enclosure volume for its drivers
3. Is properly designed for its drivers
4. Has high sensitivity (minimum of 96db to 101db or so)
5. Has a broad flat frequency response (+/- less than 3db, preferably +/- 1db) in the frequency range of your instrument, and it's harmonic overtones to the 12th multiple of the fundamental.
That frequency range would be 31hz to 524hz (B0 to C5) for a 24 fret 6 string bass, with overtones to 6.3khz; and 82hz to 1.3khz (e2 to e6) for a 24 fret guitar, with overtones to 15.6khz.
The overtones are important, but they damp out very rapidly with each harmonic doubling. The first four multiples above the fundamental are called the "presence", and they can be heard and felt, inside ones head (particularly when amplifying acoustic stringed instruments). Generally because of harmonic attenuation, it's impossible to hear or feel anything above the 8th multiple of any fundamental (and above the fourth it's essentially inaudible) but you want the extra headroom, because even the best speakers attenuate towards the ends of their response curves, and harmonic attenuation mostly damps out the quieter harmonics above the second to fourth multiple already (I'm speaking in terms of multiples of the fundamental, instead of first through 16th harmonic, because I don't want to get into the variability of harmonics with scale, tuning, time and periodicity, resonating chambers, free air space etc...).
Speaking of that, you'll note that the lowest frequency on a five and six string bass is 31hz (it's 41hz on a 4 string). Unfortunately, almost no speakers have decent, clear, uncolored response below 45-50hz, even those in high quality bass cabinets. A good full range PA speaker will generally go down to between 45hz and 55hz, a will a good bass cab (classic Ampeg SVT cabs only go down to 58hz
This is deliberate, because unless you use a giant 15" or 18" subwoofer, (or use special very expensive 10" or 12" drivers) you just can't do 40hz or below and still sound good. Even with the giant subwoofer, the bass can sound muddy and rumbly.
Better PA speakers and bass cabinets still respond at frequencies under 455-55hz, they just roll off more than 3db (better speakers might have something like -3-6db at 5hz under nominal, and -6-10db at 10hz under nominal, with around 1-1.5db/hz attenuation for the next 10hz under nominal). This actually has the effect of making the bass sound "cleaner" and clearer; though at 31hz, it's still more felt than heard.
There are a few bass cabinets that have good response in those sub-bass ranges, like the Markbass standard 4x10 (response down to 35hz), and Ampeg Pro-Neo series (28hz in the 15" and 38hz in the 4x10,), but they're very expensive (they run about $900 to $1500).
Ok... so why is a PA speaker more accurate and "better"?
Frankly, most guitar cabs flat out suck. They're inefficient, heavy, expensive, and don't sound very good.
Generally, only the very best (and often most expensive) guitar caps sound great (or often, even good), and even then only in specific situations, for specific tones, with specific amps (because they have a strongly colored and biased voicing).
Even just a mid quality PA speaker, is generally better at reproducing sound accurately, than a very high quality guitar cab. The frequency response will be flatter and the sound will be clearer, with neutral coloration and voicing.
This is because a guitar cabs are deliberately designed, and their drivers are deliberately biased, to emphasize certain tones and attenuate certain tones (mostly attenuating very high and very low frequencies, with a curved response on the mids and mid-highs). When the speakers significantly alter the sound coming to them from the amp, this is called strong coloration, or strong voicing.
In general, it's very difficult (and expensive) for an 8", 10", or 12" driver to have desirable tonal characteristics across the full range of guitar frequencies and overtones, and most cabinets don't include high frequency horns (and the ones that do, generally don't voicematch them very well, blend them very well, or use good quality crossover networks, so they often sound harsh, thin, and "squeal" or "crackle").
Again, only the very highest quality (and most expensive) guitar cabs do so well (or at all); whereas even lower end PA speakers are specifically designed to use multiple drivers (a 12" or 15" full range will typically be a 3 way speaker, and may be as much as a dual woofer 5 way) to produce pleasing and even frequency response across the entire range of instrumental and vocal frequencies and their audible and "presence" overtones.
A single high quality high performance driver for a guitar cab, might cost as much as $450, and even midrange drivers can run $90 to $150. You can see how in a cheaper 2x12 cabinet retailing for $300, or combo retailing for $600, they've got to be buying lower end drivers, that just aren't that good.
At last with a combo amp, the manufacturer should have chosen drivers that match well with the amplifier. It's nearly impossible for a strongly voiced cabinet to sound good with a lot of different amplifiers. Manufacturers have to either try to make it as neutral as possible, in which case guitarists don't want it ("it sounds dead" or "it sounds cold"... it doesn't, it sounds neutral you just don't know how to use your gear to get the sound you want instead of having it done for you by the "strongly voiced" drivers), or give it a strong voice that sounds good for the amp they think most customers will want to use with it, for the style of music they think most customers will play with it.
This is one reason why you see many artists in certain styles of music or with certain play styles, all seem to have a strong preference for the same amps and cabs. It's not just fashion (though in part it is), it's because the manufacturer specifically voices those amps and cabs, to sound good for that style of music and play style.
Ok, forget about "voice" and "color", what about the rest of "better" ?
Decent PA speakers are also generally much more efficient than guitar cabs of equivalent quality, volume, and tone; because all but the very best guitar cabs have quite low sensitivity (some as low as 82db, and 86-92db are not uncommon for cheaper cabs and speakers, as opposed to 96-101db for the best of both PA speakers and cabs).
Higher sensitivity, means that for a given power level output to the speaker, the volume output will be louder. A 3db difference in sensitivity, means DOUBLE or HALF the power is required to produce the same volume.
An aside on speaker sensitivity, volume, and solid state vs tube amps: Sensitivity ratings are given as db of sound energy output 1 watt, at 1 meter away from the speaker.
We don't hear 3db as a doubling of volume. 3db sounds roughly 10% louder to our ears, but it is a doubling of sound energy, and a doubling of input power.
It takes a 10db change for our ears to hear a doubling in volume, as well as taking 10 times the power (to double volume at the same frequency, impedance, and sensitivity).
However, because both our hearing, and the power requirements for sound energy are logarithmic, a 3db more sensitive speaker, will sound about twice as loud, if it's given 5 times the power as the less sensitive speaker, instead of 10 times the power. The difference gets even starker at 6db, or 9db and so on.
The most efficient speaker cabinet I've ever heard of was 103db. The least, 82db. Cheap combo amps will often have 86db or 89db speakers, while more expensive combos and cabs will generally 96db or 99db sensitivity. Let's see how that works out:
An 82db speaker will output 82db at 1 watt, 85db at 2 watts, 88db at 4 watts, 91db at 8 watts, 92db at 10 watts, 95db at 10 watts, 98db at 40 watts, 101db at 80 watts, and about 104db at 100 watts.
An 86db speaker will output 86db at 1 watt, 89db at 2 watts, 92db at 4 watts, 95db at 8 watts, 96db at 10 watts, 99db at 20 watts, 102db at 40 watts, 105db at 80 watts, and about 106db at 100 watts.
A 96db speaker will output 96db at 1 watt, 99db at 2 watts, 102db at 4 watts, 105db at 8 watts, 106db at 10 watts, 109db at 20 watts, 112db at 40 watts, 115db at 80 watts, and about 116db at 100 watts.
A 99db speaker will output 99db at 1 watt, 102db at 2 watts, 105db at 4 watts, 108db at 8 watts, 109db at 10 watts, 112db at 20 watts, 115db at 40 watts, 118db at 80 watts, and about 119db at 100 watts.
A 101db speaker will output 101db at 1 watt, 103db at 2 watts, 106db at 4 watts, 109db at 8 watts, 111db at 10 watts, 114db at 20 watts, 117db at 40 watts, 120db at 80 watts, and about 121db at 100 watts.
A 103db speaker will output 103db at 1 watt, 106db at 2 watts, 109db at 4 watts, 112db at 8 watts, 113db at 10 watts, 116db at 20 watts, 119db at 40 watts, 122db at 80 watts, and about 123db at 100 watts.
You'll note the 103db speaker is essentially as loud with one watt, as the 82db speaker is with 100 watts. It's also louder with 40 watts, than the 99db speaker is with 100; or louder at 20 watts, than the 96db speaker is at 100.
This efficiency factor by the way, combined with much higher gain in the preamp stages (which makes things sound louder at lower overall volume levels); is why cheap "100 watt" solid state amps, sound much quieter than very expensive 15 watt tube amps. The cheap solid state amp might have even cheaper 86db drivers in it. The tube amps have expensive 99db or even 101db sensitivity drivers.
That tube amp can get about 50% louder with 99db speakers at 15 watts, than the cheap solid state can with 86db speakers at 100 watts. With 101db speakers, it can get about 70% louder, and with 103db speakers it can get almost double the volume.
Tube amps used expensive high efficiency speakers, because they had to, because otherwise they didn't have enough power to get loud, or they would badly overheat. Solid state amps are much easier and cheaper to crank up the power on, and so they could afford to use cheaper, less efficient drivers.
Higher sensitivity also means allows a given sound at a given level, to be reproduced using less power, more accurately, with less tonal coloration or distortion (this is called headroom).
PA speakers are also generally much lighter, and often much better built and tougher than guitar cabs, even though they are much cheaper.
For some reason, many guitar cabs are surprisingly fragile. In part because they are meant to be prettier (they're often covered in decorative and relatively expensive this textured vinyl for example, vs throwaway dark grey or black industrial carpetlike material). In part it's because decent guitar drivers are VERY heavy, and can do damage to themselves and the cabs structure, if bumped about too much.
Whereas PA speakers are built to be knocked around. They're lighter, because the drivers they use weigh much less, and aren't as deep, so they can fit in smaller enclosures. These factors allow PA speakers to weigh as little as half the weight of an equivalent guitar cab, while still being built tougher.
These by the way, are several of the reason why I'm using a PA speaker for testing amps and guitars, and I'm not using my Ampeg 2x10 bass cab (I don't have a guitar cab right now). The cab is voiced specifically for bass, and in particular for the Ampeg PF500 and PF800 amplifiers (the other reason is I don't want to blow my good cab, if an amp malfunctions).
Guitars actually sound great played through bass amps and cabs by the way, particularly if you have a full range cab with a high frequency horn and good crossover (my Ampeg 2x10 PF-210he has +/-2b or less from 53hz to 17khz, at 98db, covering the full range of guitar harmonics with plenty of headroom).
Guitarists have been using bass amps for more power and a deeper "growl" tone, since Leo Fender invented the modern electric solidbody bass, and the first dedicated bass amp (in 1951 and 1952 respectively); particularly after they introduced the 50 watt 4x10 model in 1956 (setting the pattern for both bass and guitar speaker cabinets ever since).
In fact, the first "stacks" were Fender Bassman amp heads, with 4x10 bass cabinets; from way back in '60, when the loudest guitar amps you could get were Vox AC30s (30 watt 2x12. Still used by many artists today, including Brian May and the Edge. They mic them up on stage for the PA). Guitarists loved them, and bought every one that Fender could make. Even today, they are among the most prized vintage amps, easily costing $5,000 to $10,000.
In fact, guitarists loved them so much, and they were so rare and expensive, that Jim Marshall modified one to make his famous Marshall JTM45 (the first Marshall amp), deciding to use 4x12 cabinets with celestion speakers... And the Marshall legend was born.That said, the bass amps and cabs are distinctly biased for bass, and definitely do color the tone.
When I'm testing, repairing, or reconditioning an amp or guitar, I want to hear as close to an accurate, uncolored, unshaped, neutral voiced tone as possible. A decent PA speaker is the best way to do that, without spending a massive amount of money.
I would love to have a top quality 2x10, 2x12, 4x10, or 4x12 guitar cab. Unfortunately, they're VERY expensive ($600 to $2000), as well as heavy and huge. Guitar cabs smaller than 2x10, generally sound like crap, and are still VERY expensive ($200 to $600 for good ones. The cheapest are 1x8", still cost $100 to $400, and they mostly sound like crap until you're past the midpoint of that price range).
Even just a half decent 10" or 12", two or three way, PA speaker ($100 to $300), set up properly, will sound as good or better, than all but the very best guitar cabinets.
Don't think so? I'll prove it.
Guess what... Any live show you've been to bigger than a coffee house or pub (and probably a lot of those gigs too), most of the guitar sound you heard wasn't coming from a Marshall stack. It was probably coming from that Marshalls preamp or DI output (or even directly off the pedalboard into a DI box), on a direct patch to the mixer, and out to the PA system.
The amp may or may not have also had the cabinets miked up and mixed in, but stage miking is difficult and inconsistent, and you need a wide variety of mics and rigs for different situations. It's hard enough with a single smaller amp and cab, never mind a big stack, or multiples. Some artists always mic their cabs, some never do, and some vary depending on the acoustics of the venue.
Either way though, the PA system has to sound pleasing while reproducing those guitar sounds. Which means they CAN reproduce those same sounds, and if they can reproduce those sounds when sent them from the PA amp, they can be made to make them in the first place.
The dirty little secret of rock and roll?
It's really hard to get good natural sound, filling even a medium sized room, out of just a big amp and cab on stage. Most of the time, most performers (and importantly, most sound engineers), in most venues, don't even try.
On stage, amp stacks aren't for sound... They're for the show... the spectacle.
In a bigger venue, most of the time the artists don't even use ONE of the stacks for their OWN audio, using floor monitors or in ear monitors (like very high end earbuds, hooked into their wireless system, and letting them hear their tone evenly no matter where they are on the stage, as well as hearing the other band members, and crew communications).
At most, they'll have one stack miked up.
To get proper sound out of a full stack, you need to use EIGHT mikes (which are damn near impossible to set up, and even harder to mix and EQ properly), and a big acoustic shield to protect the mikes from excess reverb, echo, delay, and phase effects, and from spill sound from the house and the rest of the band; which screws up the visual look they're going for.
Here's another "secret" for you... A lot of the time, if you see an artist with big stacks out there on stage, and they want that "real stack sound"; the stacks you see on stage will actually all be unpowered dummies. Their REAL amp and cab will actually be back stage, behind noise insulation, properly miked up, mixed with the DI and turned way down, using only a few watts of power so as to not overload the mics.
This is also how Brian May, The Edge, and other guitarists that like to use lower powered classic amps, particularly combo amps like the Vox AC30; get their preferred tone, while filling a huge concert venue with sound.
If the artist wants a heavy feedback sound, they'll use a feedback or screamer effect. Either that, or they'll have a single amp with the gain turned all the way up, and a 2x12 floor monitor, or a single 4x12 cab on a riser to bring it to guitar height, that they'll walk up to for natural feedback. Even then, it will be miked, and mixed in the board, to go out to the in ears and PA.
Often, their sound guy will be watching for cues, and when they want the feedback, they'll turn that one amps mix up on the PA, and then turn it down afterwards so they don't get unwanted feedback, noise, and distortion.
Now, that doesn't mean the guitar cab doesn't sound good, and may in fact sound much better than a direct clean and "dry" signal from a preamp or DI box (if it's a good amp and cab, it almost certainly will). The guitar cab will be specifically tuned to accentuate the pleasing sounds and dampen the displeasing sounds, that a guitar and amp can make.
That however, is why most guitar specific DI boxes, modelers and the like, have something called "cab emulation", and many have "amp emulation" of specific amp types (and of course the whole point of modelers is that they do this).
These are circuits (or digital models) that when played through a clean and neutral speaker (or directly into a mixer or recording interface), emulate the way a well known amp and/or cabinet (like a marshall JCM800 through a 4x12, vox AC30, or Fender 2x12 combo... those are the most common emulations by far), will shape the tone of the guitar.
This is also why you have equalizers in any preamp, amp (and most amps with a DI can switch between equalized "wet" sound, and unequalized "dry" sound), modeler, mixer, and PA rig.
But using a PA speaker as a cab, you don't have to worry about that. We're not talking about playing wet or dry through a PA. What we're talking about is playing through your full sound chain, right up to the final drive stage.
In fact... give me $300 to get a half decent 2x12" 3 way PA speaker, and I guarantee I'll blow your $600 2x12" Fender cab, clean off the stage.
I know where I can get a 30hz 101db 2x15 4 way pa speaker for $300 (with eminence drivers even). That's stackable with a 101db 2x12 4 way in the same form factor also for $300. That's $600, in two cabs that collectively weigh in, and size up, just a bit bigger than Marshall 4x12. But sonically they would just blow the $1200 Marshall away. In fact, I'm certain they'd absolutely kill a full dual 4x12 stack costing $2500. Same amp, just two PA speakers instead of two cabs.
Between your pre-amp and amp (all guitar amps have a pre-amp. It's what makes them guitar amps rather than just power amps), amp and cab modeling or emulation, and proper equalization; you can get a good PA speaker to sound as good or better than an actual guitar cab of the same size, price, and quality (though not necessarily exactly the same as a specific amp and cab); and generally better than all but the very best cabs.