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,

Looks like you can connect it to any unused RCA on the NAD. All you are doing is using the outer ground connection on the RCA cable. Keep the volume to minimum on the .1 input on the REL.
Just as a follow up : I connected to the phono RCA but I assume any would have been ok. The outer ground was dangling. 
It however created a buzzzzzzzzzzz in the subwoofer, I had to turn the Longbow off to get rid of it.
Eventually, I exchanged my beloved and amazing NAD M2 for a NAD M32. Less powerful, but more recent and still completely digital. Same connections, but without the buzzzzzzzzzz :)
Thank you for your reaction @noromance !