Could be caused by a ground loop. That is, if you have the rest of the system plugged into one set of outlets and the subwoofer on the other side of the room into a different set of outlets, you can have a difference in ground potential. The end result is an increased noise floor and / or the potential for hum. You may also have a problem with one or more components plugged in with the incorrect AC polarity. This too will result in an increased noise floor and / or the potential for hum.
Other than that, i've seen more than a few comments pertaining to hum related issues with Sunfire subs. Call up Sunfire and ask for Robert in technical support. He's VERY thorough and will help you as best possible over the phone. Sean
PS... Any time you call a manufacturer pertaining to one of their products, have the model and serial number handy. This allows them to check and see if your model was produced before or after specific production changes were made, etc...