Yes, it will do line out at the same time as the headphone amp. The volume knob only controls the headphone volume, correct.

I can't compare it to the audio-gd but I use my D3 daily and so far it has served me well. No desire to upgrade at this time.

I just tried my 990 Pros out of my phone and while I was able to max the volume and still listen it was a decent level, one I probably wouldn't want to go over very often. Not sure how that will compare to your soundcard, but at some point you might want to invest in some sort of amplification...

I'd go with the DT990 Pros.

I have one now as well. I'm coming from a NuForce Icon-2. The D3 is much brighter, assuming brighter means that the highs are much more pronounced and detailed. My mind was sort of blown on how much of a difference there was between the two. It is almost too much (the highs), at first it seemed...