Contact customer service at Peavy for schematics. Too lartge a file to post here. Ask if there are any procedural or service bulletins for this model. For example not powering up with a variac comes to mind.
And a good inside contact at Peavey is Gene Ford - email@example.com
I can;t keep them straight in my head, but if this is what i am thinking, you could have lost your triangle wave carrier. I may be thinking of a different model though. I do see traingle circuits on the schematic.
But before we go too far, isolate the problem.
Plug a clean signal into the power amp in jack. Does it behave the same or is it clear? Likewise, connect the preamp out signal to the input of some other amp. How does the signal sound there?
And verify you are not losing any power supplies. Even the low voltages can be involved in the power amp, as the ICs there run off that.