Hi K. Rose,
Sounds like a great setup. Getting a good sub to mate with the Magnepans is critical. I would suggest you think of three options:
1) REL: I echo the sentiments of many that you should audition REL's offerings
2) If you are looking used, I would highly recommend Revel Performa B15 or B15a subs. I personally own the B15 and it is an amazing, tight, and fast sub. But, it's big with a 15" woofer.
3) JL Fathom. I hear this sub a few months ago when I auditioned a pair of Wilson Sasha speakers. The sub lives up to all the reviews.
I know there are likely plenty of other good subs out there, but having auditioned Maggies myself, these are the three subs that I think will be real winners in the setup.