Yeeesh.... this one is odd. It may be the power supply, but amplifier gain generally should not be a function of supply voltage -- though a sagging power supply would limit the maximum available output power of the amp (i.e., it would distort more easily). If the amp has a stage whose gain is a function of the power supply voltage, that would do it, I suppose (but such a stage wouldn't have very good PSRR). Anyway, checking the filter caps certainly is a worth doing.
Rick, is the problem that the amplifier distorts more ealisy, or does the sound actually get softer? Is there a volume control you can spray clean? Are there internal connectors between subsections of the amp which can be pulled apart, cleaned, and re-connected? Is the problem possibly other external gear, either before or after the amplifier? When the sound softens, does is loose more bass or treble than the rest of the spectrum? Any crackling? Any increased humming? Heat?
(Does the amplifier get heavier? Does the PSG begin to sound like a b@njo? Why do I ask such silly questions?)
Ya' know, one problem I found plagues aging audio gear, even the pro stuff, is solder joint fatigue. It can do exactly what you describe. This is particularly true of gear which is moved a lot or which as a fan which continuously vibrates it. I made many repairs on an old MCI 24-track 2" tape recorder in a studio where I used to work wherein solder joints would form hairline cracks and become intermittant. Big headache, though easily fixed by de-soldering and resoldering (if you could find the culprit joints!).
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