I'm brand new to the Pro 900, but even though it's very efficient, giving plenty of volume off the sound cards of two PCs and two laptops I've tried it with, the Pro 900 has a lot more "slam" (better dynamics) and bass control when powered with my Meier Stepdance (portable amp) - even when using the amp's internal 9V battery instead of a 15V external DC power supply.
I spent several hours yesterday listening to this combination: WAV -> Foorbar with WASAPI on Windows 7 > USB > CEntrance DACport LX > Meier Stepdance (on 9V internal battery power) > Ultrasone Pro 900
It was very impressive - much better than plugging the Pro 900 directly into a PC sound card.
If the $250 price tag for the CEntrance DACport LX is out of the question for now, you might ask Head-Fi member cooperpwc (Paul) for his impressions of the UD100 USB DAC. He actually uses this $50 DAC with a Meier Stepdance to drive $1200 IEMs (Final Audio Design Piano Forte IX).
I know of several LCD-2 owners who have used the Meier Stepdance with great satisfaction. The LCD-2 is less efficient (more power-hungry) than the Ultrasone Pro 900, but when used with a 15V external power supply, I find the Meier Stepdance to be nearly indistinguishable from the amp section in my CEntrance DACmini (a desktop DAC/amp) with a 1-Ohm output impedance mod. I spent several hours just this past weekend comparing the 15V-powered Stepdance to the CEntrance DACmini with 1-Ohm output impedance (in both cases using the DACmini's DAC section and LCD-2 rev.1). I was amazed that the Meier Stepdance can compete with the DACmini's amp section. They sound so much alike, you'd think that CEntrance and Meier Audio are one in the same. And lots of people can tell you that the DACmini CX does a great job with the LCD-2.
Currently, with your Ultrasone Pro 900, you're enjoying a very colored frequency response - it's not flat - it's what I call "FUN" - somewhat like the difference between a Sennheiser HD650 and HD600, but more so (more colored than the HD650) - with recessed mids, enhanced but still controlled bass, and a bright treble.
When you go to the LCD-2, you'll be leaving all that behind, shifting to a much flatter response than that of the Pro 900, with sweeter highs that are also darker, plenty of tight, controlled bass but with less amplitude, and mids that are to die for. The Meier Stepdance is famous for being very neutral across the spectrum and transparent - it imparts almost nothing of itself on the signal. Thus, the Stepdance will neither flatten the coloration of your Pro 900, nor warp the flatness of the LCD-2, but it can drive both of them very well, in my opinion.
I've probably told you several things you already knew, but I hope this has been of some value.