Since I last posted I hooked the center channel back up through the Onkyo receiver instead of the 990, and the 7700 started up silently and played perfectly, so now I know it's not the speaker, the speaker wire or the interconnects. I also have no startup racket when powering up the 990/7700 with the center channel not connected.

I spoke with Steve at Outlaw tech support. He said that the 990's usually very reliable, but since I've pretty much isolated the problem to the 990 center channel he's shipping out a replacement today. I'm keeping my fingers crossed. (Great tech support, by the way!)

Thanks everyone for the input.......I'll keep you posted.