You need to follow step 2
https://rel.net/blog/2017-12-27/how-to/connect-rel-class-d-gear/
I'm not running class d but I found this to be a necessary step for me.
Next, connect a long RCA-RCA interconnect from either an unused input on a preamp or receiver to the REL’s .1/LFE input. Do this even if you have no interest in theater, you are creating an audio ground, nothing more. This is true for 2-channel systems, we are not using the .1 LFE input for anything more than to produce ground.
https://rel.net/blog/2017-12-27/how-to/connect-rel-class-d-gear/
I'm not running class d but I found this to be a necessary step for me.