My 2nd M80 arrived today, happy and healthy. I've been playing ONE for 4 days now, heavy over the weekend, loud and merciless.
Comparing the two - if there's a "break-in" difference, it's not noticeable to me - or my dB meter. Loud, soft, heavy, rock, orchestral, choir, REALLY LOUD - with both speakers in the same spot (a second person rearranged them and switched cables - or not - while I sat stationary with the dB meter in one hand and the CD remote in the other).
I tried to hear a difference for two hours. That's a lot of music. Short takes and long takes, mix and match.
There may indeed be a break-in, but it's dang subtle.
If you are one of the folks who say they "Sound Like Garbage" for a month or so - wow. It may be true with other brands, or other axioms, or other pairs of M80s - but mine were good to go right outta the box.
Color me shocked ; )
(Believers, please don't bash me! I'm just one guy, one set of ears, one man's opinion. I'll outline my "qualifications" when I A/B the Axioms with the Altec VOTTS this weekend. Gotta wait til then for the cable to break in.)