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

Look in the comments section of this video: https://www.youtube.com/watch?v=cV_0G3GdPxQ

See REL's reply to the question from mark wilson. (some others also posted similar questions and the replies were also similar). Basically leave the black wires disconnected (if you get hum, some remedies are described). If it isn't clear, I suggest that you contact REL customer service.

Since the Bel Canto Evo 200.2 is a Class D amplifier, the black wires for your subwoofers' High Level cables should not be connected to the negative terminals on the amplifier. Instead they should be left disconnected to start, and if hum occurs from the subwoofers then you can either connect the black wires to a chassis ground screw, or you can run a pair of RCA cables from your preamp to the .1/LFE input on each subwoofer. The red and yellow wires of the High Level cables will be connected to the positive output terminals on the amplifier, as outlined in this video.

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.