Version differences among the PRO900s:
Ultrasone introduced the PRO900s in early 2009. The earliest units being produced were actually referred to as the PROLine 900 rather than PRO900. They renamed their entire PROLine line to PRO somewhere in 2009 so the change in name makes sense. These earliest units have PROLine written on the headband rather than PRO, and they come with a pair of white spare earpads. Now here is the interesting part. The white earpads have different surface material from the black earpads that come with the headphones. They sound different too. I don't know why Ultrasone would include a different-colored and different-sounding pair of earpads in the package, but my guess is they were trying to get rid of those old velour earpads that come with the PROLine 750. The thickness and the feel of the spare pads are exactly the same as pads that comes with the PROLine 750. When you install them on the PRO900, the 900 starts to sound more like the 750 ----- bass gets toned down pretty massively, and treble gains a lot of extra sparkle. They still have much more bass, but the pad swap brings the 900 not one, not two, but three steps closer to the 750. There's one more difference: these early PRO900s don't have the same 3D logo as the later PRO900s, which are much more common. They still have the flat logo that is essentially in the same vein as the PROLine 750. I will refer to PRO900 being produced from this period of time as the first revision. Things that sticks out: PROLine writing on the headband, white spare pads, flat ULTRASONE logo.
Somewhere in early 2010, Ultrasone started to engrave their logos onto the earcups. The logo is now 3D instead of 2D, and the surface of the earcups now have shiny reflective surfaces. They still come with a pair of white spare earpads, but other than that they are identical to all the PRO900s produced after 2011. This is the 2nd revision.
The third revision comes in either late 2010 or early 2011, I'm not sure. Ultrasone had either got rid of their entire stock of white PROLine 750 earpads, or are tired of users complaining about white 750 pads not sounding as good as the black pads that come with the headphone. The PRO900s now come with black spare earpads, identical to the one that's on the headphone. This is the 3rd and last revision.
There's also a limited run of PRO900 balanced. For whatever reason, Ultrasone thinks that there are actual musicians using these to mix and master tracks, either that or people have been re-terminating the headphones to allow dual entry, so they released the PRO900 balanced. I don't remember the exact price, but it was a lot more expensive than the regular PRO900. What's funny is that these balanced version isn't balanced from the factory. They come out of the factory as regular PRO900s, and are then sent back to Ultrasone's headquarters in Germany for dual entry modification. That's right, the only official PRO900 variant is itself, a mod. What's even funnier is that the connectors on these headphones are angled backwards. Let's not even talk about the cable these things come with. It's long, it's very thick, and it's extremely microphonic. I actually don't know what's worse. The fact that the connectors are angled backwards or the fact that cables like these actually exist and are aimed at the pro market. All I gotta say is, wow. Needless to say, this balanced version does not sell well. Ultrasone produced these for less than a year, and discontinued them in 2012. You can occasionally find NOS of these on the market. I used to own two pair, now I still own one. Due to the above mentioned problem, I don't use them. Or rather, I can't use them.