I don't know what an MT or MMT are (mid-tweeter etc.?), but I have thoughts on sub positioning.
A good reason to have subs separate from the main speakers is optimal placement of each. Main speakers can be adjusted front/back and right/left to give best imaging and smooth response. In many rooms, that will leave a hole in the bass, and subs can be positioned to fill that in.
For smoothest bass response across an extended area, use four subs (or more) in a distributed bass array. There's material on the
Audiokinesis site; they have such a system called the Swarm.