The hum in my system came from my Comcast Cable feed. As soon as it's disconnected from the cable box, the hum goes away.

I tried a Jensen ground loop isolator a long time ago, and it got rid of the hum, but it also knocked out some of the hi-def channels. Then, on a whim, I routed the cable coax through my Monster HTS3500MKII and it got rid of the hum and didn't affect the hi-def channels (or anything else). I couldn't believe it - I always thought of the Monster as a glorified power strip. FYI, I bought the Monster unit for $160 from buy.com about a year ago.

Good luck.