Does your monster power center have a place to "condition" your cable tv cord? I found that I could get rid of the hum either by purchasing a device from radio shack for about $3, or buy a power conditioner for about $250. I split the difference and went with a surge protector that regulated the cable signal as well. I found that the hum went away once I "treated" the cable signal. I prefer this solution because I don't get any reduction in analogue picture quality (whcih I was experiencing - via ghosting, snowy image, weak reception - with the readio shack part).
Try unplugging your sub and connecting it to a different outlet. That might also help.
