My impression of the Beyerdynamic T1:
It's an ok headphone I guess but nothing nearly as special as has been proclaimed by some of the other people around here.
I currently own what I think are pretty good head/earphones - Sony Qualia 010, Grado HP1000/HP2, JH Audio JH13, Audio-Technica AD2000, Stax OII MKI. Of those though, there are only 3 that I'd call "outstanding" - the Qualia, JH13, and OII MKI. The others I'd call above-average at best. I've also previously owned some other "above-average" headphones like the HD800 and Audio-Technica W5000.
Based on my collective headphone experience so far (a much more lengthy list can be viewed in my profile), the T1 seemed no better than "above-average" to me. It didn't really do anything particularly great and in some aspects it downright underwhelmed me.
I heard the T1 amped by both the Luxman P-1 and on my setup by the SPL Auditor, which should theoretically be a good amp for it. And I made sure to play just about every kind of music on the T1, though obviously due to time I did not listen to a full song, only snippets of songs. I played a gamut that ranged from Julia Fischer's Bach Concertos to metal (Megadeth, Symphony X, Beyond Twilight), classic rock (Led Zeppelin, The Eagles), jazz/blues (Anne Bisson, Eva Cassidy, Rebecca Pidgeon, Sonny Clark), electronica (The Crystal Method, The Prodigy), bluegrass (Alison Krauss & Union Station), and others like Robert Plant & Alison Krauss' Raising Sand. Oh and an equipment setup note: the QB-9's balanced output was running into the Auditor's balanced input.
I was really let down by both the bass and treble. The bass didn't have nearly the speed or the depth of my AT AD2K and the treble wasn't nearly as clean, clear, & fast as my Qualia (which I don't think is unreasonable to ask for a $1K headphone). Also disappointing but not as much was the lack of power/body/mid-bass (noticeable with the metal music) and a mid-range that lacked some tactile presence.
Someone here on Head-Fi said the T1 had a similarity to the Grado HP1000 but I'd dissent with that. I can't be more specific until I compare them directly but the two headphones definitely do not sound alike.
Despite my less-than-stellar experience with the T1, I fully intend to buy one. Might as well try it out in my system at home, and who knows, I might even change my opinion of it later. People upgrading from lesser headphones could very well be impressed by it, but I just wasn't. It immediately struck me the way that the HD800 did and still does - good, maybe even great for fans of the lesser models, but nothing really remarkable and at best a different flavor, unable to really compete with higher-end headphones.
The T1 also did not really sound like any other headphone I've heard and it left me trying to think of its closest sonic relative(s) - but I should avoid stating the models that came to mind because I could also be totally off the mark without a direct comparison.