Confirm that pins 5 and 7 are NOT connected (near 0 ohms) to each other on U9. If they are not connected, measure the DC voltage across pins 5 and 6. Measure from application of remote and for about 20 seconds.
Disable the drive circuit by shorting (solder bridge) pins 3 and 4 of the op-amp where pins 5 and 6 went from 120v to 0v after the mute delay. Does the amp still go into protect?
Drive a strong signal into the amp. Do you have audio on pins 1 and 7 of the op-amp in line with the driver transistors for the audio section of the amp?
I have found bad op amps closer to the RCA jacks.Two of them had more heat than the rest.They also cause a loud hum when i touch them.I have a similar one in the shop its a total mess.