This suggestion assumes it can be done successfully without confirming the servo wiring is correct. Maybe can disconnect the servo temporarily as high volume will not be needed.
Take advantage of some of the other features of REW. Open the Generator function. You can send any frequency to the NAD. For example, start at 100 Hz. Run only the main speakers, no subs. Listen at the MLP. Turn the NAD volume to zero. Switch the polarity of the wires to 1, only 1, of the main speakers. Turn the volume back up to the same level as previously and listen at the MLP. The 100Hz should be clearly louder with one of the wiring configurations. You can do this process at other frequencies also. I know, this is basic and you may have done it already. Just suggesting eat this elephant one small bite at a time.
Next perform the same checks with only one of the subs without driving the main speakers. Change the drive wire polarity on one of the sub drivers. Again, one of the wiring configurations should be louder indicating the drivers are in phase.
Next do the same for the other sub. Once that sub unit is phased properly, repeat the process driving both units to verify the two subs are in phase.
Then, run the main speakers and subs together to confirm everything is in phase for low frequencies. Would then be left with confirming servo wiring.