In Conclusion: Out of the box, the Mids and Highs are amazing, transparent and clear. The Bass is slightly more laid back, but I'm confident it will be more prominent after the appropriate burn in time.
Balanced Armature drivers don't burn in. Trust me, I've tried despite knowing this fact hoping for the best when I bought the Shure SE535. They don't burn in, so the only thing you can hope for is that your brain will get used to the way they sound and you simply learn to enjoy them. I also bought a pair and I'm eagerly awaiting their arrival.
They M2 are essentially for audiophiles, so are geared more for enjoyment than for reference. I think a lot of what Custom Art makes are intended for that purpose, and it's only the PRO stuff, like the 330V2 or the Harmony 8 PRO that is more suited for reference listening.
Great review! I'm glad I bought the M2. Was also considering the Ei.3 but went with this instead.