Best subwoofer below 40Hz for Magnepan .7?

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...

So JohnK, has weaponized sub's. You must REALLY not like your neighbor's! My hearing is still fine. I just wish "Allstate" had thought like JohnK. In that case I wouldn't have had to pay "out of pocket", to fix the foundation. 
       The initial hook up for the first Descent I purchased really surprised me. I had the front grill off to tweak the controls and was watching the transducer as the song playing was quite sub-sonically challenging. As I watched the high excursion on the driver facing me, the frequency passed below my hearing threshold. And in total silence to me, that's when the excursion took a turn to the violent. It was moving so much in fact, that as I jumped up and ran over to adjust the level I just knew that I was going to have three ruined woofers. Amazingly the sub has never shown itself to be any worse for the wear! Still more articulate than any other I personally have heard.
   I would try a "Weaponized", sub system though. It sounds neat. And definitely a "guy" thing. But it seems I am fresh out of enemies.

Check out the RSL Speedwoofer. It's a great value for money, and supports my Maggie MMGi speakers really well. Here's a review: https://www.soundandvision.com/content/rsl-speakers-speedwoofer-10s-subwoofer-review

Condosound...

I won’t be adding a sub to my newly acquired 1.7i’s, but when I had the .7’s I employed a small REL which, to my humble ears, meshed very well. I forget which model I used, but the driver was 8”. 

My 1.7i’s are 40hz, as well and yet I’m finding that adding a sub at this point is unnecessary.  Perhaps because the bass panel is somewhat larger on the 1.7i, perhaps because...I don’t know.  

I prefer Rel’s ease of integration. Only took me an hour or so to get it sounding well.