I'm happy with the setup but was wondering if anyone has added a subwoofer to the Auditor M's and how does it sound?
It will depend where you cross it over. I have a Rel B3 with Dynaudio C1 Signatures. Personally I cross the B3 over at 34hz (C1 rated at 45hz) to pick up where the C1's can't go. When I crossed it over much higher I hear a mid bass hump that for me does not sound realistic. What the sub adds is a better sound stage with some added weight to the mids (very minor) along with the lowest octaves. I only have a very few recordings that even have the lowest octaves but it sure does sound good when there.
Getting the timing/phasing right was the trickiest. You don't want the 'sub bass' to either lead or lag the speakers. My B3 phase switch is either 0 or 180 degrees so you may need to physically move it closer or farther and an inch or 2 can make all the difference. Later I swapped out the stock Rel cord with a Nordost Baseline made to rel spec and heard better 'low end' attack and decay.
Once that is done I can't tell where the sub is but do pick up where the C1's leave off. Is the sub necessary no but it does add that nice touch to make the monitors sound more 'full range'