Eliminate system hum

I had the same problem. Caused by cable tv coax cable. Try taking the coax cable out of the cable box. Then turn on your system and see if the hum is still there. But don't unplug the cable box from the wall. This will not do. You need to take out the coaxial cable that runs into your cable box, or whatever its first point of entry is.
I fixed this problem in my system with MONDIAL - MAGIC BOX. Bought it at musicdirect.com. No impact to picutre quality that I can detect. But the annoying hum noise is gone.
follow the link to check it out....
http://www.musicdirect.com/products/detail.asp?sku=AMONDBOX

I had the same problem with my video side causing hum.
I put a passive filter between the video side, going into my main AV processor, then into the main amp.

The passive filter takes RCA's in and out, and it's called a "buzz off", by ROLLS. You can find them in Guitar Center or other music stores, guitarists use them to cut down hum.

It works well in stabilizing my system, as I run my video side into the audio side, through my Quad amp, for surround sound.

FWIW, I tried the PS Audio Humbuster without success.