Which subwoofer?

Rel subs, in stereo, sized for your room.
Easy to adjust for various recordings and media, which is a requirement.
https://systems.audiogon.com/systems/8259The Time Machine doesn't lie.

Depends what your budget is suggest
Looking at Velodyne's new Deep Blue 15"
Or if budget allows DD15+

I happen to be one who (also) has a Rythmik F12-G paired with Maggie planars (MMGs).  Run through a Parasound P5 preamp and a B&K EX-442 Sonata (350 wpc @ 4 ohms) the sound is..............pleasing.

I highly recommend the Rythmik F12-G as a fast, musical sub.  The servo design those guys use is impressive.

Audiorusty may be on to something,  while I am in a similar situation as you, my Acoustat  Spectra 33 panels are a bit larger than your ML's , push out a bit more bass. The sub that came with the 33's is Crap. When listening to a bass heavy track,  those panels go down low, then I listen to something with a flatter curve and want more bottom. I have not pulled the trigger but I've been looking at the Schitt Loki Mini + . I've tried to be a Purest,  but with panel speakers and not so good recordings, sometimes you may need just a bit more. For $150 , I would look at this first before spending many times that amount looking for a good ServoSubs x 2...

Having run at least 10 different subs with my ML Summits I can safely say that the only subs I ever found to be fast and accurate enough for those speakers is the SVS SB 1000 in stereo. Don't waste your time and money on the rest of it I've already done the research and paid dearly for it.