Which subwoofer?

GRs OB servos are the cat meow. I use them about 60 hz down, and with MB columns phase plug tuned.
That is how I balance the weight of the bass. I like 80-280 or so..to be VERY fast.. I direct couple those drivers to BIG amps and use active XOs with full blown DSP multi xo points,  threshold, summation, on the fly correction and mic feed for OTF correction and testing..

Just so you know there is NO dampening when there is anything between the amp and the drivers.. That includes a passive XO.. NO DAMPENING.. No mater what you think.. ZERO.. My way it does.. just that simple. I'm not the only one either.. look around.. IF they will share..

I use to use VMPSs bass design, man were they easy for a passive design and DEEP bass. I still do when I need chest compressions..

They will do it..

GRs though.. just night and day how they work and pressure a room.
bdp24 couldn’t have said it better. When I coupled that system with MB columns.. I really got happy for the first time in a LONG time.. Man oh man does it work.. of course Planars and ribbons for the monitors sections.. again separate enclosures.. Lot of real estate though.. BUT you can actually get the sound perfect for your ears..

Where’s my transistor radio, go do a little sanding in the shop..
Clear coating plinths for the 124s.

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...