REL Sub Hum

Make sure the REL isn't on a different power circuit.

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.