Just got my M50s today as well. I am just running them out of my X-Fi right now. Pretty impressive.
The bass scales well depending on the type of music. Black Eyed Peas sound great and so does Norah Jones. Not a lot of headphones can do that. Some instruments can have a little too much weight to them, but nothing bad unless you like analytical. I love the detail in the treble area, not to much, not too little. Some music is recessed in the upper mids/lower treble. I was listening to some live music and clapping from the crowd sounded distant and compressed compared to other phones I have had. The Headroom graph shows a quick, steep dip in that area.
As for soundstage, I am pretty impressed. Everyone was saying it wasn't great. It only extends slightly out of your head, but what's in your head has nice separation so it feels bigger. My HD580's don't have great soundstage so it wasn't much of a change.
They do clamp quite a bit and were actually hurting my jaw. I'm sure some bending of the headband can resolve this. They're a bit heavy, but the headband feels comfortable so far. The pads get warm fast, but they are pleather. Overall not too bad.
Nice job Audio Technica. Unfortunately I picked mine up from an Amazon partner and they sent me used phones, so they will be going back tomorrow and I'll buy them from a more reputable place. Still not sure if I want to change to a straight cable. Anyhow, for the price I have a hard time believing these can be beat.