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.

Also if you really like the Maggies, don't jump on the speaker merry go round trying to replace them , because if you like their sonic signature I think you"ll have to spend a lot more to better them.  

I have no tone controls....  dont really need them . That is until I turn the sub on.   I have a nasty room mode with a huge bump centered around 40 hz.... its bad.   I could never get large floor standers to sound good in this room because of that.   So I've used a number of different smaller speakers with a sub,  that SMS 1 really tames the peak and the two different subs I've used it with have really benefitted from it.    That said if I were buying a new sub it would have to have the same type of correction that the Velodyne Digital Drive series had  , adjustable freq, slope, EQ.    

It has everything to do with the room, I agree. I’ve had some really good speakers in this room that just did not sound good due to their size , radiation pattern, required distance from wall.....etc. my last speakers were great but they sounded bloated when too close to the wall. The speakers before them were front ported and therefore worked better in this room and honestly were better. PSB M2 Platinum, wish I never sold those.....

I run my mains full range, they roll off around 60 and use the crossover to apply a crossover close to that and experiment with the freq. and slope until the bass is tight and no overlap to impart boominess