Hi Stat33,
I did get a sub for my MMG's, a Martin-Logan Dynamo, but soon right after I got it I scored a pair of Tympani IVa's here on Audiogon and my sub was too small to do any good with them so I actually never got to try it out.
I can tell you though that if I were doing it today, I would have gone for the DWM rather than the sub. The DWM is just going to sound a lot purer than a dynamic sub and sometimes it can go all the way down to 25 Hz. That's setup and room dependent so Magnepan only states the low limit as 40 Hz. But with the 30 day trial, you can try it out and see how well it works for you. Definitely the way I'd go unless my main priority was big explosions and such and I could budget for a sub that was big enough to go significantly lower and louder than the DWM but also clean enough to blend with the MMG's for music (which usually means a sealed sub).
I'm not a big believer in esoteric cables, because I think they make a subtle difference at best and for most of us, the money can buy more sound if spent on speakers, amps, and acoustical treatment. But I know there are those who feel differently. Maybe they'll chime in with some suggestions.
Anyway, for small subs on a budget, I'd recommend the same Martin Logan Dynamo I got, but you have to find it used since the newer ones are bass reflex. Among bigger subs, the REL's have an excellent reputation for blending well with Maggies. Two subs are better than one.
The DWM has been around for a while and was originally sold for use with the on-wall speakers, the Mini Maggies, and the centers. But at some point Wendell Diller at Magnepan decided to see what would happen if he tried one with the MMG's and was amazed by the results. So now they're being marketed as a DWM + MMG package. This is so recent that I only know of one person who's tried the combo, Steve Ford over at the Planar Circle.