Extended impression is that I might send the 600 back.
If I existed in a void in which I had not already owned the T90 I would be absolutely blown away to have the hd600.
That's not the case.
Notably:
I immediately miss bass. I'm not a basshead by any means but I sure noticed it when it's missing vs. the T90.
The voices on the T90 are not quite as nice. The timbre/tone of the HD600 is absolutely incredible. The voices are however, louder and more distinct on the T90. I watch a lot of tv and movies at the computer, so this matters.
The HD600 is just so damn pleasing to listen to. I'm going to actually consider this quite a bit tonight, but I tend to be a one headphone kind of person.
Clarity and detail are wonderful for the slot that the HD600 fits into, but I miss the increases in each from the T90 immediately.
I know what some of you will say "why would you think a 20 year old headphone could best a near flagship level headphone" and I didn't, but having heard for many years now about how great the HD600 is I just couldn't stand it anymore and had to try it out. I don't live anywhere near stores, or where meets take place that I would be able to hear this headphone other than ordering it.
And yes, I know "burn in several hundred hours will improve x/y/z" but realistically I don't have that much time with them and the difference would only be minor.
This makes me think the HD800 is really what I'm looking for and at some point I'm going to have to swallow that massive price tag. I do see why people love the HD600 so much.