REL Hum. Stumped

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.  

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.

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.

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. 

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!

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

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!

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

Is your system on a dedicated AC line?