If I understand your question, the NAD M3 has two pre-out jacks. They recommend the use of pre-out 1 with a subwoofer so that the internal crossover of the amp may be set according to your main speaker's lower limits. Fine tuning is done with the adjustments on your actual subwoofer.
It works as designed with no problems in my setup.
It works as designed with no problems in my setup.