My experience with Maggies is consistent with Alessandro's comments. I have a Velodyne HGS-15 servo controlled sub and it can't keep up with the Maggie's bass drivers. It just blurs everything from the mid-range down.
In my first week with the Maggies, I experimented with positioning following the tips from the manual and I'm getting very powerful bass since. The bass kept getting more powerful as I kept feeding more power to the Maggies. The lower registers really flourished after adding the Joule Electra preamp. This was a total surprise to me as I was not expecting a preamp to help so much with the bass.
I added the Velodyne sub as a test the other day, and I realized that there is no much bass that the Velodyne can add at this point. So the Velodyne sub became a nice, glossy side table.
I haven't experimented with the latest subs that Velodyne, JL Audio, etc. have to offer, but I'm skeptical that a sub-woofer can add more bass without taking some resolution from the upper bass and mid-range.
I think that with proper amplification and acoustic experimentation, the bass from the Maggies can be quite balanced with the rest of the audio band. In the current state of my system, I can feel the bass three rooms away from my listening room, with no sub-woofer.
Best,
iSanchez