Is the sub a powered sub? If so, did you try floating the ground on the sub's plug (i.e., using a cheater plug)? Otherwise, maybe it's humming because it doesn't know the words (sorry, couldn't resist that one)?
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Sounds like a ground issue if it is a power sub. I had the same thing happen with my REL sub. The REL has a third ground wire on the Neutrik cable. I had to change where it was connected to.
If not a power sub, try lifting the ground on your power amplifier power cord. Experiment around with a cheater plug.