As JohnK mentioned, your incoming video signal from cable TV is almost always the source of subwoofer/TV/receiver ground loops.
Here is a link to Axiom's ground isolator, which goes between your incoming cable-TV feed and the set-top box or your TV's cable input.http://www.axiomaudio.com/groundisolator.html
The ground isolator causes no degradation of digital or HD digital services. However, it cannot be used with satellite TV feeds of any kind.
By the way, your cable system may supply you with one of these if you complain. I've heard of one that did.