Best subwoofer cable??

Best check with REL here.  It will depend on whether you want to take signal at pre-level or high level.  If the former, then go for the Naim sub cable - it is specially slugged so the pre-amp can drive it without instability.  If the latter, REL can advise, although with Naim you need to take the high level reference signal off the loudspeaker terminals NOT the power amp terminals.