Hey, I'm on the same boat as you. Same headphone and a gamer. And same budget, haha. I'm also using a sound card, but mine is the Creative Labs Sound Blaster Zx which I bought last Monday. In my case however, I plan on using the Sound Card as the DAC. The sound card I have is not entirely enough to power the DT 990 Pro, I can tell despite being a noob that these headphones are barely there.
But speaking of mobility, you probably won't enjoy lugging this headphones in public. Considering the fact that it's fully open type of headphones. Your sound will leak, and sound will enter on your headphones. But if you're adamant, you can simply just pick any mobile headphone of your choice. A lot of people seems to go with the FiiO E17 or the E7K, but to me? I think I'd rather go with one of the CMOY (Bass Boost edition if you prefer considering your genre preference). It's a lot cheaper and should do the job.
Then for the desk, find out how the Xonar performs for a DAC. I've only researched mine, so I don't know. If it's good to go (I have a feeling the Xonar should be able to do the job), then just get a better desktop amp.
You can start your research with: http://www.head-fi.org/products/category/desktop-amps
So far, here are my choices, though:
1. Bravo Tube V3 - Cheap. Comes with 3 equalizer on the amp itself.
2. Aune T1 - Comes with a USB DAC. I'm not sure how this will do on mine, though. I'm probably going to ignore the onboard DAC and just go with the amp. I just have it here since it meets my budget.
3. Musical Fidelity V-Can II - meets budget. Need to look more info, though.
4. Project Starlight by Garage1217.
5. Schiit Magni - $100
6. NuForce ICON - $100
Oh, and I also need help on what to get ._.
Edited by Daskful - 6/16/13 at 6:25pm