A world of difference between the two amp design philosophies i would think. The guitar amps typically may have multiple gain stages (think mesa and marshall here) in the pre amp to drive the amp harder, the way the amps are voiced will be different as well with the focuss on bringing out certain characteristics of the guitar. For instance both Dr Z and matchless (both have el-34 based designs) try for a dynamic touch sensitive amp that responds to playing input in the way the amp breaks up. Audio amps/pres are not designed to break up at all. high end Audio amps will hopefully tend more towards neutral reproduction of a variety of music while guitar amps are voiced for the characteristic british chime of the vox (or matchless), the body and overdriven nature of the marshals and then you have the clean head room for days twins and the brown toned tweeds. Why the huge cost difference? One is used by a working musician who has only so much to spend on his tools while the other is typically a luxury item and can be priced accordingly. You can spend $10k on a dumble but only a few folks will do it. There are a ton of players working their fender blues devilles to death at $400 a pop used. Then there's the audiophile who will drop $10k on cables and not think anything of it (a friend of mine owns a killer high end pro studio and while he'll use monster cable for important low level lines he feels power cords and speaker cables are power cords and speaker cables...again tools vrs luxury)
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To add to Piezo's great response -
Guitar amps and speakers are intended to color the sound of the guitar, and the best are prized for their ability to do so. Hi fi gear, at least in principle, is inteneded to avoid coloration.
Also, guitar amps work with a limited frequency range, while hi fi rigs have to cope with the full audio spectrum. This makes a big difference in speakers and power supplies. Let's not forget that most guitar rigs are single channel, vs 2 or more for a hi fi rig.
The costs of engineering and building hi end, hi fi gear must be amortized over a relatively small number of buyers. Most guitar amps (except for the boutique amps that have price tags to match) are mass produced items.
Look inside an old pre-CBS Fender amp or Marshall amp and you'll be appalled at the construction quality - commodity parts wired on fibre eyelet boards, about as cheap as you can make it (I won't even mention the insides of Peavey amps - ugh!).
That some of these sound so good is a happy accident, and some design approaches cater to certain playing styles.
Class A amps (Vox AC-30, Matchless, etc) seem to do a great job of covering the transition zone between clean and dirty - these amps really respond to playing style. They depend on power amp distortion for their sound.
Other amps rely on preamp distortion (Soldano, Mesa, etc) to produce intense distortion sounds and compression that let you hold a note almost forver.
In guitar amps with tube rectifiers the B+ voltage tends to sag a bit when you hit the amp hard. This results in a natural compression that helps the amp "sing". Ditto for cathode bias (Vox AC-30, Matchless C-30, Fender Champ, others).
Solid state rectifiers in a tube amp give more edge and attack to the notes.
Many guitar amps (especially early Fenders) have undersize output transformers that can saturate, thus adding useful harmonics to the signal.
The build quality of most hi end gear is far better than the average guiter amp (though my Matchless HC-30 is pretty good).
I like my guitar amps for guitar, but I'd rather listen to music through my Aleph 1.2's.
As noted above frequency range and distoriton are two major things. A third is power. Guitar amp circuits will often be designed to maximize power. This is another thing that makes "distortion" attractive as it is inevitably what you get when you push for more brute power.
Not only is the tube circuit somewhat dfferent but the driver/speaker is too. There is always some trade-off between frequency response, efficiency, and power handling. Audio speakers tends to sacrifice all for flat response and guitar amp speakers do not opting for max power handling and efficiency.
That being said there are a lot of similarites in the designs too as it does not take much of a design change to introduce a little distortion or increase power a little. However, at least in good guitar amps, it is not a matter of just introducing any old kind of distortion.
Decent answers to your questions above could, and do!, fill a good sized book and require a bit of work on your part too. If you are interested in audio and guitar amp design one of the best sites on the web is guitar amp maker Randall Aiken's site. http://www.aikenamps.com Take a look at the "tech info".
Also look up any of the books by Kevin O Connor especially "Principles of Power" and "The Ultimate Tone"
He is another guitar amp guy who writes really clearly and his books are simplified in a good way and practical.
The first book, Prin of Power, is a great introductory book to tube amp design and, although I have no interest in making guitar amps (i do enjoy playing acoustic), I read it just because so much carried over to audio.
Indeed - distortion can be a good thing in guitar amps and a bad thing in audio amps. Distorion in a guitar amp can come from the preamp stage or the power stage or some combination of both. That said, there are also some very fine guitar amps with lots of headroom and I wonder about the similarities between these amps and the audiophile amp.
I take it no one is buying guitar amps of any type and using them for their audio systems.
Thanks for the info and references - they are very useful.