I wrote to Spatial for their recommendation.
Here is what Clayton wrote back:
The best design we have tested is the Rythmik Audio F12 Series. It incorporates a patented servo-control system that allows reaction time that sounds much better than normal subs, and it blends into our open-baffle designs very well. Two subs always sounds better, but you could start with just one unit.
Does anyone have experience with another (sealed box or OB) subwoofer that sounded better than the Rythmik F12 when paired with M3 Turbos? I completed get that OB subs would make sense with OB speakers.
Vikas