Do you have a plain old Classic? Or is there a "VT series" or "VTX series" inder the Classic name?
The VT and VTX have power amp in jacks. And if you have that, plug a guitar into the power amp in jack. Any change? And you can run a cord from Preamp out jack to some other amp to see what the preamp section sounds like.
The original Classic came in A and B versions. The A had four input jacks - two normal and two bright. The B version also had four input jacks, but it was one bright, one normal, a series, and a parallel. But they lack power amp jacks and preamp out jacks, so we can't look there.
What it sounds like to me is a noisy/failed transistor. Or conceivably a missing power sipply voltage. This is going to require taking the board out of the amp and soldering parts on it. We will also need to check voltages here and there. If this is not in your skill set, then the amp will need to go to your terchnician for service.