From an old Magnaplaner owner and soon to be new martin logan owner you need to keep in mind the basic premise these speaker types started from when adding subs to them. It will help you in the long run. Time for the "way back machine here" lol.
I fell in love with panel sound as a kid. The idea 20+ years ago, 70's, when I first got my old pans was really two fold:
1.) Flat speakers have one aligned driver point for all but any additional woofer's, if any, that are added. All sound except the very bottom is generated from a single "plane" and this minimized time differences in the sound eminating from different drivers in different locations, both vertically and horozontally. This is similar to what the old "time frames" and a few other speakers were focusing on at the same time also. Weather or not you hear the difference is up to you.
2.) Flat speakers have much less overall mass they are actually moving to produce the pressure levels of cone speakers. Ok this one is simple to see if you think about it. Less mass moving less distance should and usually does equate to faster response time for the actual sound to be reproduced. Wether or not you can hear the difference is up to you.

So when you add a subwoofer to a Magnaplaner, Martin Logan or other panel type speaker as I used to call em you need to get a very very fast subwoofer to keep up with the response time of the panel speaker. I know this sounds weird and some will surely kill me for saying this. But I tried numerous subs on my old pans back in the 70's when I baught em. Almost none back then could keep up. I could heard it clearly, here is the panel sound and right behind it was the sub sound. There was next to no real mixture of the sounds. It is a speed issue as well as a properly used crossover/amp/placement issue. The slow "boomy, fade in and out" sound of some subwoofers can really degrade that fast panel sound you get from the Logans.

I used some AR's on pedestals as rear speakers with the maggies originally. The AR's had fast and tight low bass with a smooth high treble response. As rear speakers on stands they blended with the old maggies maggies in front. Worked well 20+ years ago. But the new Martin Logans have taken a major step forward in refining the panel sound in my opinion. The Pans have improved greatly too and are still close but for me the Logans have the smoothest/speedy sound now. I am reviewing subs for the Logans I will purchase soon as my remodle is done. I am told the Revel 3k model is outstanding for the money, also the B&W. Probably others I havent heard as well. It's your call as always. But be carefull you don't kill that panel sound with a sub that just can't keep up.
I am not an engineer and I can't give you all the technical goodies to verify this. I am just an old audiophile who loves music and has seen and made a few simple mistakes before.

Take care