REL Hum. Stumped

Is your system on a dedicated AC line?

It is dedicated  

i’ve also noticed that if I move the speaker cable around disconnected from the amp, it changes the hum, almost like it’s acting like an antenna for it

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.

I had the same experience with REL high level connections,  whether a single sub or two subs. The hum was only apparent when no music was being, but still annoying. I then purchased a KEF KC62 and connected left and right channels from my preamp out jacks. No hum and more bass to boot.

Alas, I too have the hum with my pair of S/812s.  Amplifier is pure balanced, BAT VK655se, which apparently presents its own unique issues with the Rel Speakon.  

@last_lemming  I am completely ignorant of technology, so I'll never be able to reproduce what you did.   Did you shield/replace the subs original power cables??  Is there a version on the market of what you built yourself?   

Thanks.  Most frustrating, but I'm not giving up (yet), as the direct-to-amp Rel makes exquisite music, etc.