Most of the time the culprit is a ground loop conflict between the cable or satellite receiver and everything else. You could try disconnecting just that. If that doesn’t do it, I hate to make you nearly start over, but if I were in your situation, I'd power down, the start with only the speakers, amp, and 990 connected to each other and to the same power strip – no sources, no subwoofer. If that tests as quiet, power down and add the subwoofer back into the system on the same power strip. If that tests as quiet, power down and add one of your sources. If that tests as quiet, add as many sources as you can until you isolate which one causes the problem. If adding any source at all introduces the buzz, try a device that runs on batteries, like a portable CD player or iPod/mp3 player, so that you can introduce an audio source without adding any additional connections to power. If it all works through the same power strip well, but plugging something else into a different outlet brings back the buzz, it could either be a ground loop problem between two circuits or one or more outlets is wired improperly, as in a hot and neutral being reversed or something like that. If a conflicting circuit seems to be the problem and you don’t have ‘electrician skills,’ you could buy a simple three-light outlet tester to determine if any outlets are improperly wired. No electrocutions please.

There are so many permutations of possible problems that a reply here, prior to some testing, could be rather lengthy with numerous suggested fixes to every possible problem. Try some or all of the testing above and then reply in the forum again once the source of the problem has been narrowed down a bit.