I do not disagree with your observation that the T1 logs "big improvements" compared to the DT880/600. My contention is that the T1 do not preserve the tonal expression and synergy of the DT880/600--IMHO a critical omission.
One would think, and one should reasonably expect, that the T1 would convey everything admirable from the DT880/600 as a basis of improvement, but--again, IMHO--the T1 fall flat, tonally, and lapse into a technically proficient yet predominantly analytical (and expensive) piece of gear.
God willing, I will compile a scorecard, listing the T1's perceived strengths and weaknesses alongside the DT880/600's. Item for item, the T1 will undoubtedly win with more pluses than the DT880/600; but--IMHE--the WEIGHTED average of the hierarchically ordered contributing factors (i.e. tonality, ease, musicality/musical involvement) will tip the scales in favor of the lesser Beyers. : )
For me, the T1s do improve on tonal presentation by quite a bit (especially in the treble issues that I had with the DT880s and very slightly with the HD800s). But we've discussed the issues you've had that myself (and many other head-fiers) can not reconcile.
I found the DT880/600s really good headphones, but the treble seemed a bit metallic with a slight thin impression. I've described the T1s as on the warm side of neutral (I believe Skylab and Bilavideo would agree with me here); which is something you have obviously not heard. So I still have to think is there something off your your pair?