No, haven't heard the Ultrasone Pro 900. Re: the headband, its seemed quite thin, but looking at the design, I think its actually quite well designed. Minimal bulk, and it doesn't seem like there are any major stress points (ie. HD555, HD595). Light enough to withstand a drop, wouldn't want to sit on them. What gets me is this that I have a big head, and the AH-D600 fits me without any extension on the headband. Fully extended who is this headphone made for? I could see that if ones head is deflated, this headphone could pose issues.
Comparing them right now to a pair of MDR-EX600 in-ears at the moment (listening to Electric Light Orchestra 'Eldorado' DCC Gold and some Proem 'Enough Conflict'). Quite a different presentation. The AH-D600 a much fuller sound and speaker-like. The EX600 I may prefer for its neutrality, but drums, strings and timpani (or rototoms in ELO's case) just rain down upon you with the D600. On Proem, listening to the D600 the bassline move along subterranean passages, synth pads envelope, and on the EX600 the frequencies shimmer, and the micro-edit cuts of each drum hit lightly rapping your ears, the bass well defined, subdued but coherent.
Just got the EX600s back, after letting a friend try them out for a month. It definitely brings some perspective back. Thinking of selling them locally, but they are so good, so I may just sell them at price that only a head-fier wouldn't balk at!