Best subwoofer below 40Hz for Magnepan .7?

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).

If you get an Antimode 8033 you will have a more sophisticated room equalization than in most subs, and you are not restricted to those subs that have such room eq. It is very easy to use and quite affordable.

The Antimode does a detailed analysis of the peaks, and then applies precise parametric equalization with 28 or 36 filters, i.e. it reduces the peaks targeted at precisely the required frequency, and with as narrow a range and as much amplitude as is needed. No fixed frequency equalizer like the Loki can get even close to the Antimode's accuracy.

From what I understand the Loki is a Baxandall type tone control, covering a pretty wide range, whereas a system like the Antimode (or an REW corrrection curve) is a very precisely targeted precision weapon. And it is very small indeed. Mine just sits behind my power amplifier, out of sight and out of mind.

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...