i'd say it simply is more detailed and refined. the delicacy, microdynamics, and inner texture is improved. a less processed, more open sound. more relaxed and involving.
and these issues are all positive traits of the MM3, which i consider a standard setter in these areas. the MM7/Exact simply takes it to another level. it's not subtle, it jumps out at you.
when i was at the designers home it only took about 5 seconds of a very familiar demo track to hear that difference from my MM3's. he was using my same (at that time) darTZeel electronics.
Thanks!