I do like the idea of multiple fast low FS drivers that will let you cross at 3K. Depending on the roll off you might be able, or need, to cross even higher if you plan on using a 6db high pass.

One thing that I have thought that would work very well with AMTs is to build dipole arrays. The only arrays I built (took one to Fred's once) had 12 drivers with 6 per side running out of phase. They were cheap old alnico drivers, had no bottom but were almost like electrostatics when it came to voicing. Reason I thought of this is that multiple drivers using the same air in a dipole configuration are very quick. You will not get extra bass from the box, but you will stay phase coherent with the AMTs, and since the AMT is a dipole it has a weird logic. Against that logic is the speed of different wavelengths, but how much difference that would make in a room shouldn't be noticable.

As for how much raising of the SPL you will get from using 8 drivers is something I would like to know myself.