Best subwoofer below 40Hz for Magnepan .7?

REL. Martin Logan. I have a 10” REL and it’s great even with speakers that are tough to integrate with a sub. I would recommend a 12” if you are looking for just the lowest octave.

I also use a Velodyne SMS 1 EQ which I think is part of my success. I would look at some of the new subs that have built in crossover freq and slope along with EQ

Maggie’s are great but you need a really good sub to augment their bottom end. Anything sluggish and boomy will ruin their sound.

I will go with rhytmik .

The integration problem is that of room modes and not so much of the subs themselves. Dipoles do not excite room modes that much, so combining with ordinary subwoofers highlights the problem. So, for solutions look at
1 dipoles subs, to address the problem at the root.
2 multiple subs, to smoothen the response
3 room eq (e.g. Antimode 8033).

You should look at a pair of used Vandersteen 2WQ, They are fast and blend well with maggies.

How large is your room and what kind of music do you listen to that has such low tones?   Those bottom frequency waves are mighty long.  :)
This info might be helpful.
http://www.moultonlabs.com/more/taming_the_big_wave/