Rat,

Try a simple test first. Disconnect the cable tv (or satellite wire) from your system and see if the hum goes away. If so, it's the ground loop problem that Spatula and Curegeorg discuss. An isolator should do the trick. Another solution is to ensure that your CATV or Satellite dish are grounded to the same ground stake that your electrical service is.

If the hum doesn't go away when you disconnect the cable, then you have another problem such as a short-to-ground with your interconnects. Make sure you wiring is clean and keep power wires away from interconnects and speaker cable. Good luck and let us know how you make out.