Very interesting thread with lots of good food for thought for us Maggie owners. My system is a bit more budget minded with Maggie 1.6's. I also use an Onix Rocket sub. What seems to be the trick with a Magnepan (or just about any other planar for that matter...), is to figure out their roll-off point in your room. You can do this with some test sweeps and an SPL meter, but you'll usually wind up tweaking by ear anyway. I've found in my room that the real trick is letting the 1.6's handle the bass as low as they are capable, no high pass filtering. I run them full range and let the Onix merely reinforce the range from about 40 Hz down. You also need a sub that is very fast, so the acoustic suspension design of the Onix is perfect.
Keep tweaking!
Keep tweaking!