The MMGs are some of the smaller Maggies, and I would suppose would be helped in the low end department. The bigger the Magnepan gets the lower into the bass register it goes.
From the research I have seen, people either piggyback off of the speaker outs to the sub, or get a sub with a high pass filter, that basically takes the speaker inputs from the amplifier, separates out the low frequencies, and sends it on to the sub, and then sends the rest out to the speakers. There are many variations on this theme, with external boxes doing the frequency separation, etc.
As to imaging, and the sound, its all a matter of room placement and playing with them till you get the sound you are after. Being a membrane speaker, a sound wave is sent both forward and back with the membrane motion. You can either inhibit the back wave as mentioned above, or tune the distance to the back wall from the speaker, and the angle, to take advantage of the back wave and use it to improve the sound.
Adding a heavy, stiffening stand to the Magnepans is well known to help the bass production.
Sorry to spout so much, I am researching the Magnepan 1.6 for my speakers when my cat quits living in my study at home.