Ok , been listening for a few hours and all i can say is wow.
These are flat out better than my HD600 and are imo somewhere between it and the HD800.
Whats 'better' i hear you ask, tonally the 900 is more to my taste with a warmer , woodier presentation.
They are also very neutral, they are not adding much at all , maybe a little extra bass but its not really noteworthy.
Soundstage on the 900 is in a different league to the HD600, it is also more accurate than the HD800 as it does'nt have that artificial wideness that the 800 has.
Build quality is actually great, some say it is fragile, i disagree , its lightness gives that impression but its arm's and grills are magnesium and i cant see them breaking anytime soon.
I need to do some more listening, i will post a review at some point but guys dont ignore this pair of cans, they are amazingly enjoyable!