dude, i just went through the same thing last week except that my seprate amp had the hum and not my sub. I will say it is a ground loop and you should pick up an isolation transformer to break the ground loop. the guy who posts the links should be here shortly to show you what i mean. don't waste your time unplugging everything in your house like i did, i still had an audioable hum with my cable and other equipment unhooked. however i inserted the isolation transformer and it took care of the hum. the part is only 10 bucks so if it doesn't solve your problem it won't be like you spent alot of money. check out the thread tittled "power conditioners" and you'll see what i mean...
