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i'd be intersted to know as well..my late 90's fabtone does the same.

I don't know much about circuit stuff, but I do know Rats do that when they're in a feedback loop. They even stop whining when you crank the feedback. Maybe you have some weird internal feedback thing happening?

There should be a small cap between Pins 1 & 8 of the Op-Amp... see what value it is it could need increasing.

Is this while using a power supply or battery? the power supply might not be regulated or the filter caps might need replacing or enlarging (or if it's battery only possibly doesn't have one at all)

The Boost infront & Active Bass issue suggests it's an Impedance issue, a misbiased J-Fet buffer at the front perhaps (some had input and output buffers, some just input or output buffers, some neither, don't know what's inside yours) having something infront that it disagrees with, too many variables.

Some gut shots might be handy.

Just because it's commercial sadly doesn't mean it's free from design problems and age can play a factor in these things too (which means yes parts can go funny, electrolytic caps can get old and start producing problems very simmilar to yours)

Having a buffer infront will present it with an Impedance that it might find prefferable or which it doesn't want to see so it could go some way to explain it.

The battery being out the equation is useful, although is the battery fresh? A low voltage could cause it to behave strangely.

Gut shots may be useful and will go some way to seeing what version it is.