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.

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.

Hey JohnK?, I have a set of Steel framed, "French" doors in my listening room. My system at just a decent volume, makes them not only bend but shiver and shake at times. Unless of course the doors or a window is open. Then I just get the wind.