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

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.  

Thanks everyone for your help. 

REL got back to me, and here is their reply:

Thank you for contacting us, and for supporting REL Acoustics. The following instructions can be used for connecting a stereo pair of subwoofers using their High Level inputs to a stereo Class D integrated amplifier:

• Twist together the red and yellow wires of the right channel subwoofer's High Level cable, and connect to the right channel positive output terminal on the amplifier.
• Twist together the red and yellow wires of the left channel subwoofer's High Level cable, and connect to the left channel positive output terminal on the amplifier.
• Leave the black wires for both subwoofer's High Level cables disconnected to start, and ideally with tape covering the copper ends of the wires. If you experience hum from the subwoofers, then the black wires can be attached to a ground screw on the amplifier if one is available. If a ground screw is not available, then you can leave the black wires disconnected, and instead use a pair of basic RCA cables to connect from the .1/LFE input on each subwoofer to any unused analog RCA input or output on the amplifier, with the .1/LFE Level control then being lowered to the minimum position. Either option will provide a ground reference for the subwoofers and will reduce the hum.

 
I hope that this helps, but feel free to let me know if you have any additional questions or concerns.