REL Hum. Stumped

Ok solved it!

The clue was the fact when I moved the stock cable around it would pick up hum next to certain devices so I assumed EMI.  So I made shielded cable  

I bought some 4 strand shielded copper cable from Home Depot. This cable has 4 coated copper wires and a fifth un-coated with a foil sheath.

The un-coated wire that runs exposed along the foil sheath in continuous contact with it.  I soldered the uncoated wire to the black coated wire thereby creating a grounded shielded wire.  I abandoned the white wire, and used the red and green wire as the hot wires. 
 

zero hum!

Nice good job 👍. I was gonna suggest a cheater plug to float the ground. 

Had a similar issue with my S/812.  
 

For some reason on AC in my duplex outlet caused a hum.  The other did not.  
REL suggested running an RCA (assuming you are using high and not low connection?) to my integrated.  That helped as well.  
 

Now however I power the sub through my Transparent PowerIsolator (conditioner) and that has entirely solved the issue.  Frustrating.  Good luck!

As a side note do not connect the black Speakon wire to the speaker terminal if it is a balanced amp or has XLR inputs. If you must use it connect it to chassis ground ( or a chassis screw). That is cool you figured it out. I use upgraded power cables on my subs. I know some manufacturers say you have to use their cable. I think it’s a guage min. 

Please don't use a cheater plug for extended use. The work as a trouble shooting tool but are dangerous if there is a short. They are designed to provide a ground on an old two wire house not lift the ground of a three wire system.