REL Sub Hum

Make sure your power cables and sub cables aren't tie wrapped together. You need some separation between them.

I own one. Had the same issue. Simply needs to be grounded.

If the hum gets louder with volume it most likely indicates a ground loop

Try using a cheater power plug on the sub. 

I had the same problem with my Sub Utopia. 

Could also try one on the amp to see. 

In addition to the other responses, if you’re bridging the amp, it acts like a differentially balanced amp so you can’t connect up the black wire to the black post as it’s not a ground but rather an additional hot supply. You’ll need to ground to the chassis or some other secondary ground.