Best subwoofer below 40Hz for Magnepan .7?

You may want to look into the Martin Logan, "Descent i" subs. There are still some available "New" at a close out price. Three 10" aluminum drivers, Three independent amplifiers, "All servo corrected transducers", with "Room Correction", in the "i" models I've heard. I have the original "Descents" and swear by them . They are a 106lb. sealed box, triangular array that is inherently vibration/distortion canceling and well damped. With an balanced, XLR connection choice. They work great with my line source array system. 
   They can be driven to tremendous levels. They actually cracked the end of my homes foundation squarely off at the end. So I had soft expansion joints installed.

Cracked the home's foundation! Hopefully that didn't damage your hearing somehow...

Running military grade sub bass system its about 1000lbs and it doesn't crack concrete but it can bend the steel door to room when running tests. Since I know a bit amount about material science I would say expansion contraction due to thermal stress or earth movement most likely cracked the concrete foundation.  Infrasound is less damaging to hearing since it is mostly conducted by the human body it can damage eyes by retina detachment or may cause other issues our human whole body resonance is from 2-10hz  the human diaphragm about 40-60hz eyes about 16hz.

My Schiit Loki is usually in its bypass mode, but if a recording lacks treble energy (or "Fletcher Munson Curve" based late night low volume tweaking is necessary) the highest frequency (8kHz) adjuster will bring sparkle in, and really get the cymbals highlighted smoothly, and the same with the reduction or addition of bass. I rarely use the midrange bands. It might be the best 150 bucks I’ve spent on any hifi gadget as it’s really useful and I haven’t had tone controls on a preamp for a long time. My current trepidation regarding an Antimode box regards my hesitation of adding another thing in the line to the amp, and the fact that I don’t seem to need it in my listening room. But if I find one cheap enough I may still give it a shot for fun.

This is what The Absolute Sound had to say about the Audiokinesis Swarm (and it seems to make sense): https://webcache.googleusercontent.com/search?q=cache:vtIuKJVOkCoJ:https://www.nrc.nl/nieuws/2018/02...