The T1 is much more neutral sounding. When I did my auditioning it sounded veiled and my amp which is the Grace m903 did not drive them well at low volume.
The DT990 has a much needed 100Hz boost, but at times can deliver a really unbalanced sound signature.
The DT1350 is my favorite of the bunch. They are pretty neutral sounding especially for a portable. My only gripe with them is that the drivers are not always the same so the sound signature varies quite a bit from pair to pair.
What kind of sound are you looking for? Neutral or slightly bass emphasized? The DT1350 is one of those great headphones that unless you have a particular issue with you may find hard to improve upon.
I just got the DT1350, so I'm comparing it to the DT990, and I think I overall like the sound of the DT1350 more [not sure because I might be influenced by new toy syndrome]. The DT1350 sounds a lot more clear [crystal clear] including the bass. The DT990 has plenty of bass quantity but it seems kind of muddy and maybe disconnected from the midrange/highs, and sometimes it sounds like the highs are also a bit disconnected from the mids. My DT1350 sounds more neutral than my DT990. One thing I love about the DT1350 is that, when there's supposed to be deep bass, it delivers it with aplomb! Regarding comfort, on the other hand, the DT990 is MUCH more comfortable. I find the DT1350 to be moderately uncomfortable after less than an hour.
I think my hope was that the T1 would be like the DT1350 but sound bigger and more expansive.