Could this just be transformer noise -- i.e. mechanical hum? I just pulled out my old Adcom GFA555 (original version from the early 80's) for use as a sub amp. Works great but has a hum that's only audible when my system is off and it is dead quiet (except for the amp). It's not a major problem, since I can't hear it when listening, but I was planning on pulling the top and trying to tighten the transformer. Maybe lay a sheet of sorbothane under it?
These old amps are such work horses. Do they ever die? The only other glitch is that the power on/off switch just broke. But, as I've seen reported elsewhere on Audiogon, it broke in the "on" position. Even when they break, they work.