I currently use 3 Martin Logan Depth i subs with my ARC equipment. I run them on speaker level receiving their signal from my 610T's. I had tired line level from my 40th Ann pre amp but sound and control was no where near as good.
Is there a reason that you have an issue with ARC equipment?
Sorry for the late reply.
I was experimenting with running a REL sub through speaker connections and did not seem to have very good results. It almost seemed anemic for a subwoofer. It wasn't very musical. Not sure if this is ARC or REL issue. I moved REL sub back to home theater room.
REL makes some great subs as witnessed by the numerous accolades they have received. Rest assured, once dialed in properly, they are not "anemic". You should follow REL's recommendation and try the Neutrix / High Level input if possible.