Peavey Combo 300 TOO QUIET?!

[Standalone]

My assumption, since the EQ works and you get some volume, it's likely something is dirty or corroded in the power section. I wouldn't first assume its a knob but it is possible. Of my 2 Combo 300's, one needs a jumper in the effects loop to hear anything at all (as opposed to your issue), so while I hope it is a simple fix I think that would not be the primary fix here. A can of deoxit never hurts with these old guys and I hope you share in my experience that it's one of the few tools actually needed with these unstoppable amps.

Good luck and do yourself a huge favour don't leave the pre or post too high, (maybe 2 or 3) in any of your testing, once you find the issue you could make yourself deaf.

+1 I do think the jumper trick (plus deoxit) is worth trying. 100% sound loss surely isn’t the only symptom of a problem there.