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 definately going to try a different sub cable. The system is about 2 years old. I have all axiom speakers and an onkyo 750 reciever. The hum has been off and on always. I have removed and then replaced the ground screw several times. sometimes it works better with it in and sometimes it's better with it out.
the hum has been around off and on for about 6 months. just recently it has been getting louder and louder.
