NAD M23 and a Stereo Pair of REL Subs

Hello folks,

I recently purchased a Class D NAD M23 amp to replace an older Class AB NAD C272. Ive got it hooked up to a pair of bookshelf speakers and everything appears to be working just fine. The trouble happens when I try to integrate a stereo pair of REL subs (via high level connection). There are instructions from REL on the peculiarities of connecting in high level to Class D amps, and the proper way to connect a stereo pair, there was also this recent post from another member:

https://forum.audiogon.com/discussions/how-do-you-hook-up-a-pair-of-rel-t5x-subs-to-a-class-d-amplifier?sort_order=desc

Well, despite it all I can only seem to get the left channel speaker outs to activate either sub. Try as I might I cannot get the right channel speaker out to do the same. Clearly both speaker outs work since the bookshelf speakers are producing sound just fine. And clearly both subs are working since I can get either to work with the left speaker out. In case you're wondering I've also eliminated the cables to the subs as a factor.

I've contacted both REL and NAD but I probably won't hear from either for a few days. Just curious if anyone else has run into a similar issue? I'm hoping this isn't a curious artifact of this particular amplifier design.