Guitar Amps versus Monoblocs?

The Mesa Baron is the hifi amp that's closest in design to a guitar amp (consider the manufacturer!) in my opinion. I can't imagine any pure guitar amp that I know of doing well at hifi, though.

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.