It's probably a ground loop
Disconnect the cable TV completely from the entire system and see if the buzz goes away.
If so, you need a cable TV isolation transformer.
Also, lift the ground on the electrical plug...
The hardware store has an adapter that changes a 3 prong electrical plug to a 2 prong plug.
This type of adapter is used to plug a modern (3 prong) plug into a old style (2 prong) electrical outlet.
This adapter lifts the ground connection from the electrical outlet. http://www.filmtools.com/grliadco41.html
Please DO NOT cut the ground pin off the TV plug, use the adapter instead.
By LIFTING the grounds with the ADAPTER(s) you are likely to find the source of the ground loop. Leave the adapter permanently installed on the TV or subwoofer (etc...)
The adapter costs about $1
The adapter has a ground tab, or ground wire included, but don't connect that to anything. (you are DIS connecting the ground on purpose).
When an audio or video system is being grounded to two points at once, it often causes a buzz, or a faint horizontal line drifting in the TV picture.
The two grounds come from (first) the cable company.
second the ground in your house, which comes from the electric utility.
When these two grounds are used at the same time, you hear the buzz.
By disconnecting one of the grounds, the buzz goes away.
We use the isolation transformer on the cable TV, or the electric plug ground lifter, to eliminate the second ground.