Yep, already did that. See troubleshooting #3 above.

Does it sound like a 60Hz Hum? Grounding loop. I can't remember where I read this but your problem might be a grounding loop in a hi-fi/electronics system/circuit. Not sure why it would be just the center channel though...

Try plugging the 990 or the 7700 on a separate circuit totally... Long extension cord may be needed.

Here is what I found online...

http://www.epanorama.net/documents/groundloop/

I tend to think that something is wrong with the 7700 and it's interaction with other components in your system.