REL subwoofers prefer use of a fairly simple Speakon cable that connects to the speaker terminals on the amp side. You can use bare wire or attach you own termination connectors if you wish. REL's Youtube channel offers excellent videos explaining these connections to various amp types.
https://www.youtube.com/channel/UCYoAoFjONrGaZuyEjdd9nFQ
I use banana connectors from my speaker cable on the amp side into the Marantz (spades on the speaker side), and use the Marantz bare wire entry port on the same speaker terminal for bare wire from the REL Speakon cable. Both can connect simultaneously. As you probably noticed, Marantz 2 channel amps do not support LFE.
https://www.youtube.com/channel/UCYoAoFjONrGaZuyEjdd9nFQ
I use banana connectors from my speaker cable on the amp side into the Marantz (spades on the speaker side), and use the Marantz bare wire entry port on the same speaker terminal for bare wire from the REL Speakon cable. Both can connect simultaneously. As you probably noticed, Marantz 2 channel amps do not support LFE.

