Not sure on how to link to YouTube but if you go to YouTube and search, "If you only watch one REL video, watch this one"

He addresses getting rid of hum.

you have a few possible problems and solutions

one run an extension cord plugged into where the gear is also plugged in

this will tell you if you have a ground loop.

alo make sure you are using all three wires from the speakon

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. 

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.

This

How do I stop my subwoofer from humming? Watch the above post.

Once you've solved the ground issue, trim the cable(s). They are not well shielded.

Better yet, buy these. I had them terminated with spades and I did not need the black "ground" wire but you can also order a 3 wire version, and only one if you dont need a stereo pair. These are FAR better than stock and cheaper then the REL blue wire or even SignalCable.com

@audiotroy appreciate the suggestion - what I did test is getting all the equipment on the same extension cord. With your suggestion, how would one test for whether there is a ground loop? Am I plugging in anything to the extension cord? 

What is interesting is that the EXACT same configuration works fine w/o for the other parasound amp. 

@willywonka yes I did after Mr. Hunter's latest video, which did not address the issue! Power cable goes off to one side of the REL and the speakon to the other.... 

@izjjzi Yes I did see this one, was good, but didnt make any difference... 

@blackbag20 - interestingly, hum fairly loud and constant... does not change with volume.... does that narrow down anything? 

@noromance yes, they are all in the same power outlet... 

@vinylshadow by cheaper plug, do you mean power cable? 

Thank you ALL!! I have a feeling its THIS amp (A23) and maybe it has developed any issue with grounding... my other parasound (2125 v2) with exact same configuration (ground to back amp terminal and yellow/red twisted to positive amp terminal). The A23 was working fine in the past with same RELs; I had put on the shelf for a month or two trying another amp and them plugging it in, identical as before and the issue started... which leads me to believe there is something going on in the AMP... 

also @sizzitrail84 I am running in stereo mode.... 

I am going to try the RCA method, @lak which the article you posted called suggested. 

Schematics are not easily available for your Parasound amps so I can’t confirm, but I wonder if the A23 is a fully balanced design and the 2125 v2 is not.   This would explain the different results when connecting the amps the same way.

Try connecting the A23 as differential/fully balanced per the REL manual (page 15 in my Serie T/x Operating Instructions).  That’s Yellow to the Left + terminal, Red to Right + terminal, and Black unconnected or connected to ground.  This is how I connect my differential/fully balanced amps to my REL T7/x without any hum.

@mullster No. A cheater plug. It goes on the end of the subs power cable and into the outlet. It eliminates the ground prong

@mullster I have not had any issues when connecting my REL subs with the speakon cable however I did have a hum issue with a different brand of sub that connected with the speakon cable. Adding an RCA interconnect and following the directions provided in the above video I posted worked for me.