I didn't actually try and integrate the sub with the M3s. At the time I was experimenting with separating my 2ch from my 5.1. For music I used an integrated Yamaha A-S801 and an Onkyo AVR for HT. When I wanted to listen to music I would swap the speaker cables.
So my experiment was getting a Sennheiser Ambeo soundbar, which is a very expensive large soundbar, along with the SVS sub. With it's built-in digital amps I would be able to use that and the sub for movies and leave my Spatials hooked up to my 2ch amp for music.
One thing I missed with the sub was that pressure wave from the dual 15s the Spatials would put out. Sub would rattle the room and go deep but it wasn't the same large feeling you got from the Spatials. The soundbar itself dipped down into the 30hz area but still you just can't replace the displacement of the Spatials.
So that does not mean you can't integrate subs with the Spatials. In my situation I felt like I really didn't need an extra piece of equipment to get those extra few low range sounds. If you are trying to do HT 100% then yeah you'd want a sub.
From what I read here you would ideally get an open baffle sub to match up better with the Spatials. Not sure how much it would matter with HT? Probably matter more with music.
Or maybe go with the X-3s? Built in powered subs. I'm sure someone here has used them for movies.
I'm no expert just wanted to impart how impressed I am with the M3s and movie dynamics.