You many have introduced a ground loop into the system by connecting the audio system to a Home Theater system, Cable TV box, and or powered subwoofer. The cable is ususally guilty followed by the Sub which usually gets pluged into a different outlet. I installed two isolation transformers from Jensen transformers (not cheap about $ 150+ each )My background is black as night now.
Unplug the above items to see if they are at fault when you plug them back in
Unplug the above items to see if they are at fault when you plug them back in