I got my pair of M40x today and they are not quite what I expected them to be.
"Tuned flat for accurate sound monitoring" - not really, these are actually quite bassy and the sound is more V-shaped than flat.
I use Shure SRH550 DJ as my main headphones - compared to them the M40x have better build quality and are more comfortable but they lose when it comes to sound quality.
For the ATH the bass is the central instrument and then comes everything else, whereas for the Shures the bass is just one of the instruments that are all equally important and equally represented.
The SRH550 simply have richer, more spacious sound that is well balanced and truly flat, and they cost me about 40 euros less than the M40x.
If I notice some changes in sound after a period of burning in, I'll be happy to report it, but I don't really like what I'm getting out of the M40x right now.
Conclusion: the M40x are OK headphones, maybe even great if you are a basshead, but it is hard for me to recommend them to anyone when the Shures SRH550 DJ and SRH440 offer better SQ for less money.