I have both headphones. The T90 is clearly the superior headphone, being better in all aspects. It has better dynamics, is significantly more transparent, and has higher resolution and micro detail. The bass (which I regard as an area of weakness for the DT990) is cleaner, tighter, and more extended on the T90. The highs are also more extended, and is also sweeter and not as glaring as the DT990.
I upgraded from the DT990 250 ohm, and have been very pleased. I still love the DT990, and use it for "fun" listening while on the go. Its still a very entertaining headphone, very capable in its own right especially on electronic and rock music.
As for the T90, it is my reference set for serious listening way into the night. Paired with the right amp (I use the Bottlehead Crack), it never fails to bring a smile to my face.
