If you have a ground loop that causes 'hum', especially in your subwoofer, try this before you buy a line filter for your cable box.
Locate the cable connecter box that feeds your building. It may already have a ground wire connected to it and to the electrical ground for your house. Clean the wire with sandpaper and re-connect. I have to clean mine every couple of years.
If the cable is not grounded, you can buy a grounding rod assembly or run a heavy gauge [10 or 12] copper wire to any water pipe in your house and connect with a pipe clamp made for this
