Best Subwoofer for Magnepan speakers

I have to agree with Terryakhan. Properly set up in a tuned room, and provided with adequate, quality power, the larger Magnepans are capable of bass that is deep, articulate, and fast. The speed of the Magnepan panel makes for finding and mating a sub of equal speed difficult. That said, many Magnepan users have had good luck with the Sunfire and Vandersteen subs. My advise would be to not consider doing it unless you are in a position to use a pair of them.

I have custom designed subwoofer systems that I designed and built embedded in the wall behind my Maggies. I am well satisfied with results. While this design may be impractical for most people, (eg: apartment dwellers) some of the concepts I used may be relevant.
1..Every speaker should have its own subwoofer. This might mean three if you have a multichannel rig. Five would be overkill.
2..A large area of SW cone is desirable. In each of my three subwoofers, I have a 15" and a 12" driver, stacked. The idea here is that the Maggie is a large area low excursion driver, and the subwoofer should work the same way. A line array of four 10" SW drivers would be a good choice.
3..Locate the subwoofer directly behind the Maggie(within 2 feet or less), and raised off the floor so that its center is halfway up the Maggie (centered). The idea here is that the subwoofer "plays through" the Maggie. Good "integration". I have a hunch that the subwoofer works in part by absorbing the Maggie backwave rather than projecting its own sound.

just went through the subwoofer wars, spending weeks reading what everyone else thought and liked. listened to many subs. would have loved to get the velodyne dd series, but due to not being able to justify 5000.00 on a pair of subs, i settled for a pair of the velodyne spl 1200r's. gotta say once i found a happy place for them sitting back in the corners behind my system, they are a very excellent sounding sub. then again its all what your ear likes, not anyone else.