Originally Posted by
Don
My channel switching amps- a Mesa/Boogie and a Marshall- behave pretty much the same way. If you crank the drive on the od channel and turn the guitar down there's very little breakup (the Marshall gets muddy, the Mesa sounds good).
It's natural for even a dirty amp to sound clean when the guitar's turned down. It's also natural for a cascading gain channel to make noises when both the drive and master are cranked.
Still, if you bought the amp used, it's a good idea to have it checked out by a reputable shop.