I have three pairs: M50, M40X and Sony MDR-7509HD.
Basically to evaluate a sound, you have to compare it to other audio equipment.
My main reference sound source are Behringer Truth b3030a's studio monitors (before you judge do a research), and some Polk-Audio RTi A3's speakers.
The two speakers sound phenomenal and extremely balanced for their prices.
Let me start with the Sony MDR-7509HD:
In comparison with quality speakers and the ATH-M40/50, they sound like a castrated toy.
Too honky (2k broad bump) with no treble above 7 KHz and no bass below 100 Hz.
Plain awful... the typical Sony headphones sound signature.
Heck, even the Sennheiser HD201 which I bought for my half deaf father (he was in a tank crew in the army) for TV watching without disturbing the neighbors, are better sounding than them MDR's.
For a long time I thought they sounded good until I bought my first real studio monitors, the Truth B3030A which took me a LONG time to break-in to them because of the excess treble and bass or so it seemed at the time.
ATH-M50:
A huge difference and step up from the mdr7509HD's, I was blown away in aw as to how can a pair of headphones can sound so similar to a good studio monitors.
To me they sound "In you face" with no overpowering frequencies, or annoying/irritating single frequency that stands out from the rest and grab you attention.
After hundreds of hours (or thousand) I am so familiar with their sound, they ARE what headphones should sound for me.
I bought these in 2011, they are my main cans to date.
ATH-M40x:
Bought a month ago (March 2015), and boy they sound fantastic.
In comparison with the M50 they have a little more spiky treble or just a tad different.
But, the low end is about the same as the M50, although I don't have the M50x but according to reviews and sound tests on SonicSense, the M50x are less balanced than the M50 or the M40x, they have boosted bass.
The M40x are less "woody" than the M50 because of the tad smaller driver size, but it makes them sound even closer to a speaker than a headphone.
After getting used to them (I don't believe in burn-in) the treble spike disappeared and they sound extremely flat and neutral with no remorse on recording mistakes,
they can easily replace monitor speakers once you are familiar with their sound.
These are great headphones and I would be 100% satisfied to stay with the M40x if my trusty old M50 broke, without upgrading.
I have no issue with comfort on all three of them cans whatsoever.
As to sound stage, all three are closed back and have no sound stage. I don't see how a closed headphone can create a sound stage at all, IMO you need speakers in a room in-front of you for that.
It's a psycho acoustic issue and I don't demand sound stage on closed cans or any cans, I learn to work and listen with an in-head experience that headphones naturally give.
One last thing,
I urge people to go to SonicSense.com (link:
http://www.sonicsense.com/resourcecenter/app/audio?load=featured&preset=over_ear_headphones&source=result and "test" headphones before you buy.
To me the M40x sounded closer to the real recording than many megabuck headphones in that list.
Once I received them physically, I can confirm that they are actually are one of the flattest and most neutral headphones you can buy in comparison with studio monitors (speakers).
Cheers.
EDIT:
Second/added opinion from closer listening tests.
The M50 are more mid-bass heavy than the M40x and may sound too up front in comparison to monitor speakers.
The M50 definitely have some "fat", "honk" and "wood" in them; Again, apparently because of the larger driver size, but that makes the M40x even better/closer to my speakers than the M50.
It may very well be that 40mm drivers are the sweet spot for my (everyone else?) ears & head.
The more I listen to the M40x the more I like'em over the M50... I feel I might make a switch in the very VERY near future if I continue to listen to them.