Like I said before, the 12AX7 does a bunch for the tone of this amp. I've never actually played through an all-tube amp before (it's on the to-do list, though!), but the Marshall AVT50H blows away every solid state amp I've ever tried. Just for reference, I play this through a matching AVT412 cab (another great piece of equipment).
I've never had any problems with noise coming from the amp itself--quiet as a mouse, really. If anything, there might be some noise from my pedal chain, but that's not the fault of the amp. There's a small exhaust fan--to keep the circuitry cool, I imagine--and I suppose if it's totally silent, you might be able to hear it. You don't buy a guitar amp to be silent, though...never heard the fan before, if you catch my drift :-).
Most of what I play is contemporary Christian worship music and this amp handles it extraordinarily well. I probably play clean half the time and with overdrive the other half and both sound great. The spring reverb adds a nice touch of fullness too. No pedal can quite imitate the sound of a true reverb tank.
I've only messed around with the drive channel once or twice, so I can't say much about it, although it's not your run-of-the-mill built-in fuzz or distortion. It actually has a nice overdrive sound to it.
I bought this thing used and I'm not sure if the original owner had any problems with it, but it's worked flawlessly for me. Sure, there's been some minor stuff--I think one of the plastic feet on the bottom of the head fell off and there's a few scratches on the Tolex, but I've never had a functionality problem.
This particular amp was made in England--not China or Taiwan--and it's well-built. Definitely NOT some cheap piece of garbage. I surmise that I'll probably have to replace the preamp tube on of these days, but it hasn't quit on me yet.
I originally bought this amp because the deal was just too good to pass up--a Marshall head and a matching cab for just $400. Not bad, considering the head and cab go for $400 each separately! Anyway, I wasn't really sure that I'd get much use out of it when I bought it, but now, I'm glad I spent the cash. This is, without a doubt, the best sounding amp I own.
I've been using this amp for about two years and I've grow more fond of it every time I plug a guitar into it. If it was lost or stolen, I would definitely search out a new one, or at least something similar. I don't think Marshall makes this particular model anymore...really a shame, since it's an excellent amp!