Just got a new sub (M&K MX-150THX) and it's humming. I unplugged the RCA cable and it still hums when only plugged into the power outlet. Tried plugging it into different outlets around the room without the RCA cable attached and am still getting the hum. I plugged in my old Polk sub and it produces no hum. From the threads I've read using the search function, it doesn't sound like a grounding problem, but I admit that I'm clueless about the whole thing. Any suggestions? Is my new sub whacked or what?