Kenwood KR-810 occilation

I have a Kenwood KR-810 that has an odd problem. It sounds fine, unless the treble adjustment is all the way up, then it starts having a high pitch squeal and getting distorted. The squeal is around 15Khz. It also does this if the volume is over about 4.

I tried replacing the output module (STK-460), main filter caps, and cleaning the pots to no avail. Should I check coupling capacitors? I do have the schematics for it, just thought I'd ask to save some time. It's not the best sounding receiver, but I'd still like to fix it.