I would also check the main filter caps on that unit to make sure they aren't leaking. Some amps of that era were prone to it. Also check the emitter resistors on the board that feeds the outputs for cracks or open resistors.
I also unfortunately had to part out a D-350B recently. I have 2 of each module. I don't know if they are any good and I don't know how to test them, but I will help someone out if you need them. They are still on the board, so I can mail someone the whole board. Contact me if you can use them.