There is no point for improving the amplifier in your system if you do not detect differences between a high-end amp and a built-in sound card. The most important fixture in a system should be the source/DAC, especially with cans like TH900/D7000/Ultrasone Edition 8, etc. I have to say, I started my head amp journey with the following:
E7 -> E9 -> Burson HA-160DS -> Lyr -> DACMini -> SPL Phonitor,and at each point I noticed differences between said amplifiers. The differences were very easily discernible when using a headphone like the HD800, but that is a 300 ohm can and when under-powered you should definitely feel that something is"not right". With the TH900, a low impedance and high efficient can, you will be noticing smaller differences at each junction, but they are there if you listen closely.
Another impression of the TH900s:
After using the TH900s for a few days now, and producing some music with them I have to say that my mixes with the TH900 are really good and easily translatable to speakers. The bass is always the hardest part to mix when using headphones, but the TH900 actually does something right in this regard (something the D7000 wasn't when I used them a couple of months back, my mixes always were with overblown bass when I played my mixes on speakers). My favorite can for mixing thus far was the HD800, but now the TH900s beat them. Also the music is more enjoyable when producing and examining elements in the mix, something the HD800 didn't allow that much for.