One of the best places to search for this hum is in the video section of your system (TV, DVD, VCR, etc). While the sub is on with no music just hum unscrew your cable TV from the wall and see if that does not get rid of your hum. I had the same problem and that was it. I just happen to have a ground breaker for my cable and now it is gone. Another thing that works great is one of those filters that you buy for your car system to keep the alternator noise out. Put that on the problem and that will work. Of course, you will loose some of your sound quality with the cheap interconnect wires on it but you will have NO noise. Hope this helps and if someone else said these things already forgive me.
