I would try some extra/changed C over preamp low-
voltage supplies or try some chokes in the pre rails

If these fix the problem, then the problem must be something
fundamentally
wrong with the receiver's design. I suspect that isn't it.

At one time it was very common to use 1uF tantalum capacitors to bypass
various devices because they would go into oscilation or produce high
frequency hash without them. In the 1970's they were "required" for both
input and output sides of three terminal voltage regulators.

Over the years most of them have failed, some with a loud bang, some with
no
noticable effect. It's quite possible there are a few bypass caps that
worked
perfectly fine when the receiver was built that no longer do anything at
all.

No argument! But that wasn't what the poster said. (See above.)