The two most immediate things I noticed with these cans were - the heightened bass response & how easy these were to drive from a volume perspective. What I would normally go full volume on regular gain on my DT880s or halfway to three o'clock on high gain, I can achieve with my T1s all on regular gain, and nothing, not even shoddily mastered or live recordings (particularly bootlegs) pushing it to regular gain at its highest extent, let alone high gain.
Coming from my neutral-bright DT880s, the T1 particularly caught me off guard at first, but a re-listen of my favourite album of all time, John Fahey's Fare Forward Voyagers (Soldier's Choice) (1973), made it all clear - both figuratively and literally. It's that heightened lower-end, providing Fahey's bass notes with an almost drone-y feeling, hypnotising me for its forty minutes. I don't find the mids recessed - they expose weaker vocalists and/or music mixed with lesser emphasis on the vocalist, and allow the instruments to take over instead, or place on pedestals vocalists that wish to highlight their unique vocals. One quick listen of tracks from Joanna Newsom's The Milk-Eyed Mender (2004) is a perfect example of this - I've never heard her vocals being as intimate and defeated on Clam, Crab, Cockle, Cowrie (2004), or as authoritative on Peach, Plum, Pear (2004). The treble isn't as sparkly as my DT880s (although not much really is), and honestly, it's a nice change of pace. If the treble response on the DT880 are a hailstorm - constant, attacking and hard, the T1's treble feels more like a cloud - you could stay in it forever.
And in terms of literally every other aspect of sound - detail, soundstage, instrument separation - the T1 is far and away the better headphone in that regard. This is my first high-end headphone, and it may not be my last, let-alone an end-game one, but I'll cherish this one. I'm interested to see how the DT880 performs once my Sansui AU-101s come - and my T1's compatibility on that integrated amplifier.
(P.S. The DT880s feel more comfortable for now, but they've had three weeks of wearing in compared to my T1s just arriving yesterday. I'm sure that over multiple weeks and even months, the comfort of these cans will be fully-realised, and maybe might brighten up a little? Although I love them as is.)