REL .LFE input

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. 

I don’t know the answer to your Speakon dilemma, but I do believe it is inadvisable to use a Y adapter from stereo pre-out to LFE in, nor would I connect one channel. The correct solution is a buffered summing circuit, but you’re going to have to DIY, if all else fails.

Have you tried this:
 

I just got two T/5x subs and got them set up no issues, that doesn’t help you but the rca trick on the webpage above might.