Best subwoofer below 40Hz for Magnepan .7?

Excellent advice and ideas…gotta love this forum.

My listening room is 20’ X 28’ X 9’, and it has an additional 7’ behind the speakers with stairs to a lower level. I have no boomy issues, so perhaps the stairwell acts as a trap for low frequency reflections (that’s using my second set of speakers, which have SEAS 10” bass drivers and extend below 30Hz).

I agree that RELs and JLs are fast enough, while big and slower drivers (15”) might not work well. I’m going to demo Martin Logan’s 210 in a few days, and I’m interested in the Vandersteen 2WQs, which are new to me. Also interested in Antimode 8033 which I don’t know at all.

The Maggies deliver plenty of detail, but in my (new) experience they need to be played loud to bring it forward, so I may rethink this, as some have suggested here.

Thanks everyone.

"The" best? That's easy: the GR Research/Rythmik OB/Dipole Sub. A dipole speaker with a dipole sub---how ya gonna beat that?! And one employing servo-feedback ta boot!

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.