NAD M23 and a Stereo Pair of REL Subs

SOLVED- 

It turns out this is a result of NAD's design (for the M-series of eigentakt amps at least). Someone with an M33 integrated ran into the same issue. I'm posting the response they recieved from REL below in case this helps anyone else:

"...it is not only a Class D amplifier but it also inverts the polarity for one output channel in order to maximize the efficiency of it's power transformer. As a result, the following instructions should be used when connecting a stereo pair of subwoofers to this amplifier using the High Level input:

Combine the red and yellow wires of the right channel subwoofer's High Level cable, and connect to the right channel negative output terminal on the amplifier.

Combine the red and yellow wires of the left channel subwoofer's High Level cable, and connect to the left channel positive output terminal.

Connect the black High Level wires for both subwoofers to the ground thumb screw, located on the amplifier's back panel near the power cord inlet."

good job

self help at its best...  👌

Since one channel output inverts phase, doesn't this mean that with "normal" connections that the main speakers would be out of phase resulting in the main speaker's woofers canceling each other out?  

THANK YOU.  Been struggling with this too.  Is there somewhere to read more about this?

So. . .  would both subs have the polarity switch in the same direction or opposite directions? 🤔

Just feed the subs from regular rca line out from your preamp. I have a REL and found no improvement in using the high level outputs of the amp, and it's much simpler.