Best subwoofer below 40Hz for Magnepan .7?

You should look at a pair of used Vandersteen 2WQ, They are fast and blend well with maggies.

How large is your room and what kind of music do you listen to that has such low tones?   Those bottom frequency waves are mighty long.  :)
This info might be helpful.
http://www.moultonlabs.com/more/taming_the_big_wave/

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