Thanks Will. I have tried everything you suggest, all to no avail. There are 2 ways to do a reset (including pulling the AC power) and I've tried them both. My gut reaction is that a chip or circuit board responsible for the multi-channel output has failed in some fashion. My gut also tells me this is an extremely rare occurrence.
Since there's nothing wrong with the laser pick-up or transport mechanisms, the machine works fine when playing ordinary DVD's or CD's. However, that's not what I paid $700 bucks for. I hate the thought of having to buy another machine just to play my SACD's and DVD-A's, especially one with lesser quality DAC's than the Marantz, but if I can't fix this, then I guess that's where I'm going to have to go.
I was hopeful that Marantz engineers, given a complete and accurate description of the problem, would be able to at least point me to a chip or board that might have failed (I can't be the first person in history to have a problem like this, and I'm capable of replacing parts like that), but didn't expect to run into a brick wall of refusal when I asked them for help. It has certainly colored my opinion of that company, and made me reluctant to purchase any more of their products in the future.