So
I just got this amp back from my tech, finally...
let me tell you, at first I was skeptical about this being a Quantum model, as was my tech: he has never seen one of these before, but his guess was that it would "not be very loud"
Well let me tell you he was wrong! This thing is amazingly loud... I would say it has almost as much volume and presence as my modern SWR350 amp... wow...
It sounds very modern, clean, warm without being muddy, and a massive amount of treble is avaliable, just as described in other posts. I haven't cranked it to 10 yet, but it got wall rattlingly loud, without overdriving!
The only downside to this amp is that the caps are leaking, creating a lot of buzz and hum that can't be removed (even hums on standby). Unfortunatly to replace the caps, and essentially rebuild the entire amp, would cost much more than I paid for it, and much more than I have.
But it does indeed sound amazing!