[codefox1]

Have you checked the bias in the output stage? I would also replace the coupling caps in that stage, and if there is a small capacitor across the primary of the output transformer, I would replace it too. You pretty much have to take these components out of circuit to test, and it's cheap and easy to just put new ones in. And clean the volume control too. If that doesn't do it make sure the problem is not before the output stage by tracing the audio with another amplifier. Could also be a problem with the phase inverter circuit or tube. I don't have the schematic in front of me, but that's the course I would follow. Good Luck.