What subwoofer to use with Maggies?

Keep the woofers small,in multiples.I would suggest using a 6db/oct crossover into the MMGs.Will you be bi-amping or passive?Can the woofer assembly be placed in the same "plane" as the MMGs or possibly used as a base?
If a box can be built,the Audax Aerogel 8" drivers are being cleared out at $25 a pop.Their efficeincy is 89 and might be used in a passive arrangement.

Forgive me for asking,but do you use a 5.1 system for muovies/2 channel for music? The .1/LFE sub is meant to deliver bombastic explosions and dinosaur stomping.The "music" sub does its' job without notice.
When dealing with the "music" sub,it is usually designed to work with a particular speaker system,or it needs to be flexible enough to work with multiples.The first can be done on a moderate budget,the other is more expensive to implement.

Any number of woofers can be used in a series-parrallel arrangement.Using 4 (2 squared) will appear to the amplifier as one 8 ohm,unless you are looking to lower the impedance-ie:4 ohm.