Humming sub.

You're right Eddie, there's a very very low hum left but not like it was before.

Mine has a tiny bit of hum. However, once there is a signal into it, the hum disappears. Check your cabling to the sub. It is picky about interconnects and speaker wire.

The amp may be causing the hum if too close to the sub? Isolate the amp from the sub.

Final note: Play with the volume control.

Once you lessen the hum, never mess with it again and you'll have a killer sub. I never even turn mine off.

The amp is 12 feet away.

Make sure the cables don't cross over anything else.
Try plugging the amp into a different outlet? Mine really likes a line conditioner.
Definitely try different interconnects/cables. Move the sub closer if you have to just to try different cables.
My guess is that the amp & sub hum even when the pre/pro is turned off?
Playing with that much power can cause issues...

I sent this trick to a friend who had a humming Sunfire sub that many said needed to be internally rewired.
Here is what did the trick.
Take a thin run of plastic coated wire from Radio shack, find a screw on the metal plate of the sub and attach one end of the wire.Now take the other end of the wire and see where in your system the hum stops as you touch the chassis of your components.When this didn't work I told him to touch the screw on the plastic plate cover where his sub was powere at the wall outlet. I did this to get hum out of a Phono stage.Bingo, no hum from the sub.