I got these last week and will try and get a formal review together.
In a word, amazing. Wait, no. Amazing sounds like one would get sonic thrills. I would describe them as unimpressive in their accuracy, meaning that they don't excite and thrill at first listen, but you eventually realize that you are listening to some very accurate and flat response/natural sounding cans.
I haven't even put the batteries for the active electronics/noise cancelling because I do not intend to use them in that manner.
Response is very even. Bass is there, digs very deep but does not bloat. It has a nice sense of power but is not goosed in order to do this. It appears when called for. Mids are uncolored and natural, as is the treble, which is whatever happens to be on the recording. On first listen, I thought the M4U 2 was a bit warm and slightly rolled off on the top. Tried another CD and there was a clear, open and slightly bright top end.
Imaging also sound a bit odd at first, but it seems the drivers are so well matched that centered things like vocals sound like they are hovering over your head. You almost get the effect that there are no boundaries (sides and top) and the music just resides in the manner that the recording specifies.
Another thing the M4U 2 specializes in is scaling....orchestras and strings sound big, music surges, ebbs and flows naturally. A total lack of any sort of compression that a headphone might do to the music.
I A/B'd against the Denon D2000, which sounded like a bloated murky mess, yet with a bright top end tacked on top of it. Yuck. The Focal Spirit One competed somewhat in the mids but was midbass-heavy. A couple of others sounded quacky in the midrange.
So it's just the first few days with them, but while I am at work, I long to get back to my PSB headphones. I can't wait to hear what all my favourite music will sound like on them.