I would start systematically unsoldering and resoldering every joint on
the
board. It's not enough to melt the solder and push it around. You need to
suck or wick it off, then apply fresh solder, preferably eutectic (63/37).

To the OP. I have had a number of problems with bad connectors on Denon AV
amps. Just because the connectors 'look' good, I wouldn't immediately
dismiss them as being blameless. It would be worth squirting some switch
cleaner in, and then 'working' the connector in and out a few times. Also,
if you have the facilities / expertise, its worth reworking the joints on
the surface mount DSP ICs

I appreciate the suggestions. There are about 70 pins connecting the
Audio/DSP board, but I think I'll go ahead and re-solder each of them.
However I had to remove about 100 screws to disassemble this thing, and I'll
have to pretty much reassemble it to test whether the resoldering worked.
But what the hell, I'm retired!

Ken