How do you hook up a pair of REL T5x subs to a class d amplifier?

Yes, you can run wire from Sub 1 to Sub 2. That assumes your ground situation works. Good luck with that. I’ve never had the “leave the black wire off” solution work on differential (in your case digital) amps. You can try the other methods, they may work. I had the RCA cable to LFE for ground work once. You will most likely wind up using pre outs-RCA. Honestly, it doesn’t really make much, if any, real difference. 

Yes, thank you, I found that. My question has to do with hooking up a pair of subwoofers, do you hook up both of the speakon cables in parallel to the amplifier terminals?

Connecting in parallel would be like using a single subwoofer, but doubling it up with a second subwoofer. You would be sending the same signal to both subwoofers. This is a valid approach and some people believe it is the best way when using more than one subwoofer.

However, what REL describes in their comment is connecting as left and right stereo subwoofers. One subwoofer connected only to the left channel and the other subwoofer connected only to the right channel. For a class D amplifier, connect red+yellow to the + terminal, nothing to the - terminal, and black wire not connected. REL prefers using stereo mode for two subwoofers.

But seriously, send an email to REL if you have questions or doubts.

+1 @mspot . He has explained it well. But if you are still uncomfortable, contact Rel

Perfect, thanks!

0. [Connecting a subwoofer to the Sphinx:

   The Sphinx has one pair of variable outputs that can be used to drive a subwoofer. The variable outputs are buffered in order to eliminate any interaction between the subwoofer circuitry and the Sphinx.]

My first call would be to the amplifier manufacturer. The output stages of switching amplifiers can vary dramatically.