The HICAP was never designed to drive a long interconnect, so any longer than around 2m can cause instability and poor sound as a consequence. Naim offered a 3m or 5m pre-level interconnect for use with their own n-Sub - this had a 100R resistor in it to prevent instability. However…
The REL way using a high-level connection is a good one (and in many ways preferred to the pre-level option), except that Naim amps can be upset if you take a feed directly off the amplifier output terminals - that’s because they don’t have extra inductance on the outputs but rely on the speaker cable itself to provide the inductance for stability. Better with a Naim power amp is to take the high level reference feed from the terminals on the back of the speakers, that way the amp doesn’t “see” the subwoofer connection. If you use the REL cable you may need to split it so you can reach the back of both speakers or you could make up a new custom cable that’s suitable.