I own the T1, and have previously owned the DT880. They do indeed share a sound signature and are very close in that regard. The T1's have superior imaging, depth and is more revealing to my ears. The T1 has a better extended bass response with better definition. I'd say the DT880 (600) is 80-85% of the T1 in sound quality, at least to my subjective opinion, others think both more and less in improvement. The T1 is nowhere close to 2x as good as DT880 ultimately for me. Only go for the T1 if money is of no concern.
Interesting and good to know. Thanks for the the comparison.