REL .LFE input

I did. I tried floating it and connecting it to the grounding lug on the amp. 

Also wanted to share this post from a user on audio aficionado, it could be the speakon connector internal wiring:

“I’ve been disappointed with the stock 10m Neutrik cable that came with my REL T-zero sub. It’s a great little sub for a medium/small room, and serves duty in my 2.1 home office setup. The cable originally came mis-wired, but I did not know. It took a lot of troubleshooting to find my initial hum was located at the Neutrik connector. Then it involved a lot of finicky fiddling to get it in the correct position, with the cord laying in the perfect position so the sub would fire but not hum. Eventually I got fed up enough to take the connector apart. There were three wires, and four Phillips head tiny screw clamp connectors for the tinned ends of the wires. I immediately saw it. Wow, that’s the culprit! Loose wiring, especially the yellow wire. There were four possible locations for the wires, and the three wires. I double checked their locations. The yellow wire was almost dangling loose in the wrong place. Flopping around in there. I corrected the wiring, and made sure the connections were snug. I fired up my system, and lo and behold she worked beautifully. I really love this little sub, but I just didn’t trust the cable.”

I had a similar issue with my REL T9x. Was emitting a hum at around 60 cycles but only when using the vinyl rig. My connection is also the hi level input. I ended up replacing the wire with a set made for me by bluejeans cables. They do make a custom set that is very nice for the money. Problem went away so there may be credence to checking the cable itself. 

Cheers. 

Reaching into the cobwebs in my memory here, but,

Doesn't REL have some precautions or special procedures  for accommodating high-level connections on Class D Amplifiers? Perhaps you've been there and done that already.

Your Technics SU-G700 is called Class D by many reviewers, but Technics insists that it is "digital" not Class D.

Anyway, some REL support on this sounds appropriate. 

Good luck to you.

I did check with REL, they gave me the same responses as in the video that was posted above, which I tried, and yet has not worked. I will look at the cable though and the internal wiring and such, that seems like a good place to look.