--Update 4 years later!--
There was a lot going on back then in my life, with job, baby, travel... and I let this amp sit for awhile without turning to it. Then my smaller Denon PMA-300V amp recently bit the dust (protect mode, probably a failed power transistor... I have another thread on that, I think it's a lost cause) so I decided to turn back to this one
I opened up the relay. It was easy to pop the cover off it -- just squeeze the plastic sides and it came right off. I used a business card cut into strips, soaked in Deoxit, to clean and sand the contacts inside. And I adjusted the idle current with a multimeter while I was in there based on the instructions in the service manual.
The amp works!!! I've been using it for a few weeks now. It sounds beautiful, actually. I'm really pleased with it. Excellent clarity, separation between instruments, depth, and a spacious, three-dimensional sound. I actually can't believe it's working!
I really want to thank all of you, belatedly, for all of your patient advice in this thread -- especially Karl, JHB, mbz, Binkman, NAD, redk, and the rest of all the greatly helpful people here!