Hey last_grimm, I would recommenced the Sennheiser HD58x's for a solid alternative to the DT990's for gaming. The HD58x's have less emphasis on the lows, and highs, with a somewhat similar mid range, with maybe a very slight bump in the upper mids. This results in the 58x' sounding "warmer" than the DT990's, and should be a solid alternative, particularly if you find yourself being fatigued by the 990's for being too bright. The 58x's also have very good imaging, I'd say very comparable to the 990's. In games, this translates to very good directional audio, so things like identifying enemies locations based on footsteps will be fairly easy, as well, stereo sound cues will be reproduced accurately. Even with the slight dip in the highs and lows the 58x's offer fairly good detail in both ranges. The highs aren't as clear and detailed as the 990's of course, but are a lot less fatiguing. The bass while not having as much extension, and less quantity, offers solid quality, and feels very tight.So bass response should be good enough for all but the most die hard bass heads. Also the 58x's require less amplification than the DT990's so you can easily run them without an Amp if you needed to, but they still can benefit from one.
I wouldn't reccomend the HD6XX's for gaming. While they aren't terrible for gaming, the 58x's offer a better experience in that context. The 58x's and the 6XX's are very similar sounding headphones, to the point that some people can't easily identify the differences between the two for music listening. Gaming however is one place where the differences are far more apparent. The 6XX's have a bit more emphasis in the mid range, with a bump in the mid bass, but offer a bit less sub bass, as well as slightly recessed highs compared to the 58x's. For gaming audio the additional sub bass, and treble the 58x's offer should provide a better, more balanced and impactful overall experience. I'd say the most important difference however, is the imaging. The 58x's have better imaging than the 6XX's, so it can be a bit harder to accurately judge direction and distance based on audio cues alone with the 6XX's. The better imaging with the 58x's can lend itself to a more immersive experience, and even have a bit of an impact in gaming performance, particularly in high levels of play in FPS's. Also the HD6XX's, much like the DT990's require an Amp to reach their full potential. That said the Soundblaster G6 should offer enough power for either headphone.
For context, I own and use all three headphones, and these are the conclusions i have come to after extensive use, and A/B testing. If you have any questions, please feel free to ask, but I do think the 58x's should be the next headphones you should audition, as I feel they will fill your needs quite well.