If you haven't made any changes to your system such as adding any new electronic components or cables and cheated the ground on the sub I highly doubt it is ground loop issue that is causing the humming. Does the sub still hum when there is no connection to the receiver, just plugged into the wall outlet?

It could very well be that the amp has kicked the bucket. Call support.

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne