Audiophiles hifi appreciation thread old and new

Oh, and try it with the direct button depressed also, as that bypasses the balance circuity (in case that's an issue).
You should be able to work out what is/isn't an issue by doing all of those things to start.

Next recommendation would be open it up. I think you've already said you've nuked the switches and dials with contact cleaner? After that, look at the RCA jacks and speaker connectors for loose solder joints onto the circuit board. Also check to see there isn't any obvious capacitor issues (leaking or bulging).