Connecting a NAD M2 to a REL 212/SE

Dear all,

I have a NAD M2, a digital amplifier (class D). I recently obtained a REL 212/SE to get some more *oomph*.
Connecting these two seems to be challenging.

REL recommends the High Level connection with their own cable. As the NAD M2 has voltage on all speaker posts, I merely connect the yellow and red wires from the REL cable to positive L/R binding posts. The black wire from the REL cable remains floating. This, however, just generates a huge hum.

As per REL instructions, I probably should use a RCA cable to connect amp & sub to reduce the hum.On which post do I connect it on the NAD M2 site? It's often mentioned to be a phono socket, but that doesn't seem to be present.
The REL i

Here you see the rear of a NAD M2. Do I connect the LFE connector on the 212/SE to Digital Audio > Coaxial > Out, or rather Analogue Audio > L or R single ended?

Thanks for your help,
Koen