Something all of you may have missed, and an easier fix than any listed.
The exact same thing happened to me in my HT when I added seperate power amps to a receiver, what the problem was was having a dimmer swich on a lamp (or wall) in the same line. The amount of noise made by a dimmer switch is often not picked up by a receiver, but will be when seperated power amps (as in your sub) are connected. Power conditioners didn't clean it up either. Switching from a lamp with a dimmer to one without immediately stopped this problem completely!
If this is the problem, it's simple to fix-kill the dimmer switch, if not, you may need to advance to some of the other rec's above....
Good luck!
The exact same thing happened to me in my HT when I added seperate power amps to a receiver, what the problem was was having a dimmer swich on a lamp (or wall) in the same line. The amount of noise made by a dimmer switch is often not picked up by a receiver, but will be when seperated power amps (as in your sub) are connected. Power conditioners didn't clean it up either. Switching from a lamp with a dimmer to one without immediately stopped this problem completely!
If this is the problem, it's simple to fix-kill the dimmer switch, if not, you may need to advance to some of the other rec's above....
Good luck!