REL High-level connection.

[audio_engr]

How about the wiring for two subs?

From you other posts, it appears you have a Devialet Expert 120 and a pair of REL S/812 subs and your main speakers are Focal Diablo Utopia. 

So here's what you do.

1. First of all, SAM should be OFF. 

2. Each REL High-Level input cable has THREE wires; Red, Yellow & Black. 

3. [ A ] For your LEFT Channel REL sub, join the Red & Yellow wires together, and Black separate.
    [ B ] For you RIGHT Channel REL sub, join the Red & yellow wires together, and Black separate.

    [ C ] Join the Two Black wires together.

4. [ A ] LEFT CH REL sub : Connect (Red & Yellow) joined wires to Devialet 120 LEFT POSITIVE SPK OUTPUT TERMINAL
    [ B ] RIGHT CH REL sub : Connect (Red & Yellow) joined wires to Devialet 120 RIGHT POSITIVE SPK OUTPUT TERMINAL
    [ C ] The TWO joined Black wires are connected to GROUND terminal post. 

E N J O Y !!

P.S. Your Pair of REL S/812 subs are supposed to operate as mono subs; hence you will have one for Left Channel and one for Right Channel; just like your speakers. If you do the above connections properly, the full system will operate as Left Speaker with Left Sub, Right Speaker with Right Sub.