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.
If not a power sub, try lifting the ground on your power amplifier power cord. Experiment around with a cheater plug.