Best subwoofer below 40Hz for Magnepan .7?

Used RELs…I have 2 that cost 200 bucks each, and they're great…a Q150e (10"), and a Q108II (8"). Killer, go well below 40 hz (note that the fundamental tone of a low E note on a string bass is 41hz, although higher harmonics are also there for your enjoyment).

Nothing wrong with any of the above responses, but I would recommend you get another loudspeaker rather than trying to augment the .7s.  I recommend either larger Magnepans or Eminent Technology 8b loudspeakers.  My reasoning goes that the a good sub really won't work as well or cost much less than buying a different loudspeaker.

+1drumsgreg,
Even a pair of 2w's will work wonders. And, they are built like tanks. Even my ancient pair of 2w's work as the should after 20+ years.
B

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.    

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.