"I would look at those normal/bridging and stereo/parallel-mono switches,
as well as the level pots for the inputs."

I have been at this a whioe. Yes I checked and the input is getting all the way to the amp board itself, which contains all pretection circuitry. I would have switched amp boards except for what I found which is coming in a "supra" post. (not a toppost but closer to it)