Which subwoofer?

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.

Two Rels S/510

The new x series from REL are incredible. Check out my T/7x review on their website www.rel.net 

I agree about looking at REL.  They are the first subs that integrate with my Maggie 3.7s.  Very fast and great quality.