Hi majik,
I would agree with others here that you absolutely have a ground-loop hum. In almost all cases, it enters your system through the cable-TV or satellite system feed, so try disconnecting the cable at the input of the cable box. The hum should go away.
Here is an excerpt from my article on eliminating hum:
http://www.axiomaudio.com/tips_eliminate_hum.html "Disconnect the incoming TV-cable or satellite feed to a set-top box or to your TV and the A/V receiver. If the hum disappears (and you don't use a satellite dish) complain to the cable-TV company. They may know what you need and supply you with a ground-isolating transformer. If they don't know what you are talking about you'll need to order a video ground isolator and install it in-line with the TV cable before it enters your set-top cable-TV box. Axiom has special wide-band isolation transformers that will not interfere with any digital TV or HDTV signal. (N.B. You CANNOT use one of these with a satellite decoder box.)"
Regards,
Alan
