I agree ... it can be a ground loop
If you don't want to waist time connecting and disconnecting to determine what is causing the hum ... get one of this
http://www.radioshack.com/product/in...entPage=search ... connect it between your receiver and subs ... if is a ground loop it will desapear ...
...
One of my subs started humming after I added my Buttkickers amp ... I connected one of this, and problem solved