Connecting Subwoofer’s to Willsenton R8

You can use the Neutrik Speakon 3-lead cable that is delivered with most REL subs (or buy one separately) yellow and red to the positive speaker terminals on both channels and the black to either channel negative. I have a modded-upgraded R8 and use a Rel T7i (one sub). Works very well.

Thanks for info. Can I use 2 subs with that hookup ?

OP, for pair of subs, use the supplied high level Neutrik Speakon 3-lead cable as below,

Twist red and yellow wires together of LEFT REL sub and connect them to the L channel positive binding post. And connect black wire of left REL sub to the L channel negative binding post. Repeat these steps for your RIGHT REL sub / right channel.

In some instances, twisting red and yellow wires together may result in exceptionally high gain (output) from the RELs. If this is the case, please remove either the red or yellow wire from the twisted pair and use electrical tape to mask the loose wire. This will reduce output by half and restore a natural dynamic.

Can the same wires be used at the speaker binding posts instead of at pre/amp ?This would reduce cable length needed. 

Yes. 

REL sub's will work with essentially any amplifier/integrated amp since they connect directly to the binding posts.  My question (and not to hijack this post) is there a way (ie easy internal mod) to pull the variable signal off say the output of the volume control and repurpose one of the input RCA's to act as a sub out?  I have a modded R8 which I greatly enjoy.

A simple solution for streaming is to get a device like the Bluesound node which includes a sub out. Maybe they have phono amps with sub outs as well, but having sub connectivity at the source level would be ideal.

I added a speaker to line level adapter I got from Crutchfiled (link below) and use it to connect my RSL Speedwoofer to my R8. Works great.

https://www.crutchfield.com/p_543ADP12/Russound-ADP-1-2-Speaker-level-to-Line-level-Adapter.html