REL .LFE input

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.

I had the hum issue on class D monoblocks. I followed RELs procedure for Class D fully differential amp connection and it hummed like crazy. I then connected using both leads (+ and -) and it’s dead quiet. There’s some risk though as REL says not to do that.

I thought the .1 connection had to come specifically from a receiver with.1 output as it bypasses the internal xo in the sub, so you can’t connect from a pre out on a standard preamp to the.1 input. Maybe I misread that part in one of the posts above.