Is the subwoofer brand new? How long have you had your entire setup? Have you added any new components or cables to your system? Has the hum always been present?

If you can borrow another sub cable off someone I would also recommend swapping out the Monoprice cable. I have never used a Monoprice sub cable but with other cables I have used from them the build quality can be hit or miss. It is always possible the shielding could be inadequate or the cable defective which is inducing hum. You seemed to have tried everything else.

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

-Max Payne