I dont have the m7, so i wont be able to give you a comment. But wether it will require a sub or not, that also depends on how much low end you crave.
One of the good thing about sub is that you can EQ the bass and having a better ideal placement.
I always like it with the sub listening music, the bass is just simply tighter (rythmik), but also require processing for adjusting distance.
Another thing about the m7 is that i while ago i contacted Jim and inquiring wether it can be built with an open back and he said yes.