Well I completely agree with pmbuko: it's a matter of faith. The reason is that there are a lot of things we don't know how to measure. Like soundstaging and resolution. Graphs of horizontal dispersion and noise floors do not prove anything, though they are useful.
The Axioms may well have a shorter or less substantial break-in period than other speakers, especially if they were designed that way. I would assume that the manufacturer would understand the characteristics of their drivers over time, especially if they manufacture the driver itself.
One reason I started this thread was not so much to convince everyone that they must believe in break-in, but rather to keep people open to the possibility; when I comment on the changes I have heard, I don't want them to be dismissed out of hand because people feel that break-in is logically impossible. I think it's important to never let your theories overrule your empirical or common sense. This leads to academic, synthetic answers which may or may not be accurate. Have you all seen Internet accounts claiming the moon landing was a hoax? Sure looks convincing when you read those pages!
M22 Break-in Observations
As I have rapidly approached 15 hours of solid use of my M22s, I will give my break-in impressions. One thing that surprised me was that after the first few CDs, the overall tonal balance seemed quite neutral. The unnatural high-end sizzle that I noted on my first CD, Wildflowers, was not very noticeable after the first 5 hours. Classical recordings sound completely neutral above the lower frequencies, as they don't seem to demand a huge amount of transient energy from the treble. Nonetheless, when I crank up the volume past "normal" levels, that sharp treble is apparent on loud cymbal clashes and the like. I believe the tweeters are broken in for lower volumes.
As for the bass, I finally turned off my subwoofer to get a better feeling for it. Definitely strong at some frequencies, but there are still some points where it seems to be missing some weight. Maybe even in the upper bass, above some room resonation frequency. So I feel that the bass may be less broken in than the treble.
Question for Axiom engineers
Regarding sound measurements: how do you measure for frequency response as it varies with dynamics? That is, I'm sure my M22s measure very flat at a fixed volume with warble tones; my Paradigm Mini Monitors sounded bright in comparison. But I strongly believe that a quick, loud, high-frequency transient would be disproportionately loud or harsh. Maybe there's a similar effect with the bass.