@bimmerman2 Talked to Rel today, yes you can have the black wire floating (you should tape it with electrical tape to avoid it accidentally coming in contact with the chassis and causing a short.
if you have a ground stud on your balanced differential amp (NOT the negative speaker terminal as that would be disastrous, you can also connect it to that. Rel recommended doing so to me on the phone today, but it’s not necessary if you don’t have any problems with ground hum.
Back to my issue, after much diagnostic troubleshooting this evening I found the problem. Turns out the position of my left sub is in close proximity to a wireless device ( I think it is a router) in the suite below, despite the thick soundproof flooring. When I move the sub either forward or backwards the noise changes to a low and constant hum. Move it far enough and it goes away entirely.
My next step is to see if Rel thinks it is worth it to source some inexpensive EMI shielding material. My thought is I can lay 1m x 1m of it (or more) it on the floor l, and then put a small rug over it so it doesn’t look bad, and the Rel can then sit on top of the rug. Not sure how effective that mesh EMI shielding is but I think it’s worth a shot.
I’ll talk to Rel tomorrow again to see if they think this is worth trying. I’m just really glad I didn’t have to send back a defective unit and have a replacement shipped. My wife was afraid that sound was going to break the houses windows and now the kids are absolutely terrified of my subs. Rel, you are not helping with my popularity in my house! :)