This leads me back to the opening comment about the MM3s and their ability to let you just enjoy the music.
Congrats. I know exactly how you feel.
The speakers you mentioned having owned in the past do look like a solid progression from "hi-fi" exciting speakers towards more natural balanced sounding speakers. It seems you have found one that suits your taste best. It is nice to find something that plays everything well!
I am not sure I would agree however about the great value...to me a used pair of DAL V's still epitomizes great value. The MM3's dispersion pattern may be a lot better than the DAL ....is this perhaps why you perceive such a natural sound and "disappearing" act from such a large tower? Just a hunch, but the baffle looks very carefully designed to eliminate side edge diffraction and the D'Appolito design will limit the vertical response greatly.
I am also curious if you tried such large speakers (like the DAL-V's) in only an 18 by 17 room? Do you have significant acoustic treatment or a PARC? I am astonished that straight out of the box without tweaking they give you a flat frequency response in this room.