My friend just gave me 5 Soundstream M-1 monoblocks for free because they were just taking up room in his house. At this time I was only going to use two of them to power my two passive subwoofers in my bedroom. I connected them to the subwoofer out on my Pioneer receiver. When I turned them on I got a humming noise coming from the subwoofers. Does anybody have any suggestions on how to eliminate this?