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.

you will need a high quality sub no cheap home theater subs
I recommend REL's highre end subs, Bag end or ACI All are fast enoughfor the Maggies and very smooth to blend in

the new Rel smaller subs, mabye 2 depending on room size, or a Velodyne DD-12. I have an SVS cylinder that I love.

The JL Audios are excellent, and bit on the higher priced side.

Agree with the suggextion to keep the sub or subs SMALL! Eight inch or smaller.
I use a single B&W SW300 (I found used for $200.) and it is just right for my B&W 805S speakers.
Too big and it will never blend in.

Well, I am using a Sunfire True Sub Super Jr. (on loan
at the moment from a buddy)to great effect with a
pair of Cardersound Madisons, so I think if it can
keep up with them, it should work great for Maggies
as well.