See if you can find the Xonar Essence for $150 and put that extra $50 into your headphones and you can get the PRO2900. I just got off fallout NV and I had to pick my jaw up off the floor. The game is filled with ambient sounds that I'd been missing out on. I was just wandering about simply amazed at how great it sounded..... and then a car exploded. The bass just about knocked me out of my seat. The amazing thing about the PRO900/2900s are that they have amazing bass, but it stays out of the way of the other frequencies. Its the best of both worlds for immersion and positional audio.
I don't know about the AD2000, but I found the AD900 terrible for electronic music and not so good for metal. It fared well in rock. For the best gaming performance you really need an open can. Angled or displaced drivers are also a huge plus(Ultrasones/ Sennheiser 5xx series).
From my brief time trying the Senn 598 I can tell you it had a massive, airy soundstage and it fared well with electronica. It was too laid back for metal.