Does your sub have a ground loop screw at the back. If so, try unscrewing it to see if the noise can be lifted there, assuming it's a ground loop issue. If that does not work and you think it is a ground loop, go to www.jensentransformers.com. I purchased their sub filter and it works great. Pricey but there products are top grade.

Axiom stuff, Denon stuff, & Sony stuff