With regards to the buzzing via rca connections on my cable box i tested it on my blu ray player and got the same noises. I went ahead and ordered a little ground loop isolater like people use for automotive audio setups. Will test with this next week. I am hopeful it can eliminate this issue.
Do you have actual cable or is it satellite? If it's cable, I'd suggest you try isolating the cable connection to the cable box with something like the Jensen VRD-1FF. The audio transformers they use in those cheap audio ground loop isolators are pretty crappy. The most common cause for the noise you're getting is due to the cable company's lines being tied to a separate earth ground. And in those instances, the best way to isolate is to isolate the cable connection to the cable box.
Whichever way you choose, good luck!