I must've bought mine over 1 1/2 years ago.
In the space of that time, I've listened to them ... about 3 times.
The Ultrasone 900s are very spacious sounding: I use my Sony HiMd minidisc and a TTVJ Milletts (my favourite combo for source and amp) to drive them. I usually use a pair of Senns HD25-I IIs' which are my all time favourite travel headphones.
Firstly - the Ultrasones do not seal off the external world anything like the Sennheiser HD25s. Although the manufacturer says that their design enables lower volumes, this is not the case, if you are travelling in busy places, which requires you to turn up the volume, in order to block the sound out. Whereas the Senns HD25s ... seal off sound better than most closed backs (are there any better... other than a Victorian head clamp?)
The Ultrasone sound is very 3-d - the sound stage does indeed sound open. But it is totally the wrong combination for my TTVJ Milletts tube amp. It makes recordings sound 'live' ... if not, live, from the back end of a concert hall.
My other amp, is a Headamp Pico Slim, limited edition, which I've used about twice.
Paired with the Headamp Pico Slim, the Ultrasone Pro900s fare much better in this combination for me.
That is - both played, a maximum of 3 times, for less than 3 hours, and left pristine in their boxes for some museum collector to come along in the future to exclaim what beautifully designed, and desirable pieces of head kit, but never used ;/