Best subwoofer below 40Hz for Magnepan .7?

Check out the RSL Speedwoofer. It's a great value for money, and supports my Maggie MMGi speakers really well. Here's a review: https://www.soundandvision.com/content/rsl-speakers-speedwoofer-10s-subwoofer-review

Condosound...

I won’t be adding a sub to my newly acquired 1.7i’s, but when I had the .7’s I employed a small REL which, to my humble ears, meshed very well. I forget which model I used, but the driver was 8”. 

My 1.7i’s are 40hz, as well and yet I’m finding that adding a sub at this point is unnecessary.  Perhaps because the bass panel is somewhat larger on the 1.7i, perhaps because...I don’t know.  

I prefer Rel’s ease of integration. Only took me an hour or so to get it sounding well.  

somegeoffguy.....What speakers do you have now ?

Also, I have never heard a downside in using a miniDSP or similar in a sub only situation.

Since mini dsp goes between pre and power amp, it requires A to D to A conversion of the entire signal...which for someone who has invested $$$ in a nice DAC, is a tough pill to swallow.