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.
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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 !