Well you were right, IC720 is part of the problem. (snip) In the meantime why are the surround channels not working correctly?
Don't know really but I'd be surprised if all was not well after you get that IC720 replaced.
When the receivers were still pretty new, Denon tech support could only say the DSP board was bad, but they didn't stock them so if under warranty the receiver just got replaced, otherwise the customer was on his own.
Much later, during some slack time in the shop, I was able to troubleshoot as I said before - looking at the appropriate op-amp outputs, then referring back to the chip that was driving them - IC720. Since then I've replaced at least 1/2 dozen for the same problem.
Other than this and the front panel tact switches, the receivers are quite reliable in my experience.
The tact switches are important, though. They can cause a variety of symptoms, no response to a source selection, wrong source selected, lock-ups, etc.
By the way - here is the reset procedure. It is undocumented both in the user manual and the service manual, if I recall correctly.
1. Unit unplugged
2. press-hold A/D , F/P, and Dimmer.
3. Plug in, hold til unit turns on.
Unit is reset if initial volume is -75db