Best subwoofer below 40Hz for Magnepan .7?

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.

Unclemode? For 150 bucks I highly recommend a Shiit Loki EQ, as long as you don’t need a balanced thing…or you can loop it…in any case, unlike a digital EQ that’s on all the time making decisions for you (unless it’s off), this thing has 4 bands to be used whenever you feel like it, and the lowest pot, centered at 20 hz, is a very useful lumpiness extractor...you get 4 knobs to play with so it’s also a useful cymbal enhancer and mid weirdness adjuster. Also can be used as a paperweight, a really bad flashlight, and if you remove the top that can be used as a mediocre ashtray. It’s extremely quiet with zero switching noise or bothersome snoring. I use mine sparingly but hey…it’s a cool gizmo. Schiit should pay me for this plug but do they? NO…damn...

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.

I had some .7's and had the same issue you are talking about. I had one 10" Klipsch sub from the early 2000's and bought a matching sub for it on ebay for $140. I put them in opposite corners of the room and they filled in perfectly. They wanted a really low crossover too, I had them at a little under 80 Hz. I also had an Outlaw Bass Controller (ICBM) to pull some of the low bass of the .7's. I think that helps too.

FWIW, still ended up getting rid of the Maggies, they just couldn't hit hard for the music I liked  (Rock and Roll).

I agree on the Vandersteen 2Wqs.  I have had a pair for a number of years and they are excellent, and a good match for any speaker that reaches down to 40Hz.  Easy to place, easy to dial in and because they take the signal from your amp's speaker jacks, they will mate well with your main speakers.  I would recommend eventually upgrading to the outboard M5-HPs crossovers, as I did notince more transparency with them as opposed to the basic and less costly in-line filters.