I checked trouble shootin in the shop manual on the psr9000, which may be about the same,as the 8k only thing I could find that's not been suggested is a plug left in the aux send (right)on the back, will block the sound on the right?? probably not it! But check out that jack (possible stereo phono plug could have damaged it or somehow so's its not grounding out.)

I'm not very knowledgeable on solid state
electronics, but years of trouble shooting on electrical and mechanical devices taught me to look first at what the history of the device is ie whats the last thing done to it, or what was going on at the time. Seems This board has been apart a couple of times recently if my memory is correct and if I've got the story straight. Seems like there was something unplugged or something, My thoughts are remove the back and take a good look at all the plugs, and move every wire to see if a ground or something may have been missed in reconnecting....Just a thought, usually something very minor. (that 8000 is built like a sherman tank) Hope you find it........Cheers....Pose