Actually, the in-ceiling M3 is more of a version 2 of the speaker.... Ok, maybe a V1.5
The current in-ceiling M3 has a tilt-able tweeter, where the first go around didn't. So you can aim the tweeter towards the listening area.
I went with on-wall M3s for surrounds, and then just mounted 4 of those on the ceiling as well. I like the clean look of the in-ceiling versions, but I didn't want to put 4 good sized holes into my soundproofed ceiling where the master bedroom is directly above, so I went this route because I too believe that timbre matching is important.
I wouldn't be too worried about the in-ceiling M3. Axiom has been known to spend a lot of time fine-tuning to get things "just right." Heck, look at the Axiom Air. It was about 16 months ago when I visited the Axiom factory and was doing some blind A/B tests while I was there to give them feedback on what I was hearing. One of the "speakers" was a prototype configuration of their current speaker layout for the Air... It wasn't their first prototype version, and it was far from the last version. It sounded great when listened to, but wasn't quite up to par when A/B compared and you could hear what the target sound was (just a different speaker cabinet configuration).
Axiom painstakingly works to get the speaker sounding "just right" both in an anechoic chamber, and in a listening room.
However, all 3 brands you mention make good products, and if you are going with an Anthem product to send sound to those speakers, ARC is really amazing. Light years ahead of other room correction like Audyssey (and I like Audyssey). ARC should be able to make it pretty close. Audyssey and other similar products probably not so much.
