I agree that the best solution is to keep CATV separate, but for a variety of real world reasons mine isn't. But I've had excellent results in effectively eliminating hum/buzz -- and other problems -- in my audio system using the ISOMAX VR-1FF isolation transformer from Jensen Transformers (www.jensentransformers.com). It's specifically designed to squelch CATV ground loop problems at the source and this piece is more effective than the Radio Shack piece by an order of magnitude. It costs $49, but is well worth it. CATV connections can cause all sorts of complex ground loop & even feedback problems. This item is a simple & effective fix.
Cable TV causing system ground problems!
After responding to another thread about powerline products. ..I wanted to post what you might call a warning...or at least a "heads-up". Cable feeds...be they for TV or cable internet..or FM, can cause real problems for your stereo. The ground on cable can have DC and other problems..that if connected to your system through interconnects from unit to unit, or to the AC can cause you to go searching for a cure. The real cure is to keep cable feeds out of your system and your systems AC.