You've run through a good battery of tests. Changing speaker wire and even speaker didn't impact the issue. Disconnecting either end of the center channel pre-amp cable (990 to 7700) caused the problem to get louder - presumably popping even without the signal. Swapping cables so that the left signal went through the center channel of the 7700 and on to the left speaker caused the problem to move to the left speaker. The first test confirms that the problem isn't due to the speaker wire or the speaker. The second and third would seem to indicate that the 990 is not at fault. The culprit appears to be the 7700, if I had to point somewhere. The only tests that I can think of that don't involve sending the 7700 to Outlaw for some bench tests and service would be to (1) make sure the center channel's balanced/unbalanced switch is set correctly and (2) try a different interconnect or kind of interconnect between 990 and 7700 (either just replace with same kind of cable or switch from unbalanced to balanced - or the other way around, of course, if you already have balanced).