Update: Received replacement 990. Hooked it up. Same scary-sounding buzz from center speaker when 7700 is powered on, lesser but similar buzz when powering 7700 off. Obviously my faulty 990 deduction was......er, faulty. So much for my troubleshooting skills!

Today's troubleshooting efforts:

1. Hooked center speaker cable to old cheap speaker which I
don't mind blowing!
2. Changed from unbalanced to balanced IC's between 990 and
7700. Buzz.
3. Swapped center and surround back left IC inputs to the 7700.
Hmm......buzz STILL from center speaker. Interesting.
4. Changed IC's back to normal connection. Swapped center
and SBL speaker cable out of 7700. Hmmmm...buzz now
comes from SBL speaker.
5. Repeat #3 & #4 with different speaker cable, trying both
bananas and bare wire. Same results.
6. Repeat all of the above with 990 powered off when the 7700
is powered on. No difference. Buzz.
7. Connect my old Onkyo receiver's center preamp out to 7700
center in, and my DVD player's center IC output to the
Onkyo's center DVD input. Silent 7700 startup, no buzz,
center channel sounds good when I play a DVD.

Now, since my track record so far isn't good I'm hesitant to even posit a new deduction. But I gotta. It seems the problem (DC?) always comes from the 7700's center speaker output.....but ONLY when there's an IC input from the 990 (any channel) to the center 7700 input.

If anybody's still awake after trying to follow all that, I'd appreciate your opinions. Of course, I'll be humbly crawling back to Steve in TS with the news that the 990 swap didn't make a bit of difference. I sure don't want to have to try swapping out the 7700......I'm still bent over from the last time I had to lift it!