Pros: Bass extension, mids, soundstage, separation, comfort
These come so close to being my ideal headphones. These headphones have nearly everything I want, good controlled bass that extends well to very low frequencies. The bass is also present but does not dominate. Nice mids that are not recessed, a great wide soundstage and fantastic separation. The comfort is supreme, the earpads surround the ears well and the T1 is also lightweight. However, where it fails IMO is in the treble. Without applying EQ it's just too bright for my tastes.
I listen to music using lossless files and watch movies/TV shows using Bluray so the quality of the files used is excellent. People might say that my DAC/amp combo is not up to scratch, that might be so. I'm using the O2/ODAC combo which is supposed to be very transparent and uncoloured and I have read it should allow you to hear the true sound of the headphones. If this is true, the uncoloured sound of the T1 is too bright. The only flaw IMO but a major one for me since I am quite sensitive to the treble.
In terms of power, discussions in the sound science forums indicates that technically there doesn't seem to be much indication that the O2 amp cannot supply sufficient power to the T1 (http://www.head-fi.org/t/707169/can-the-o2-amp-fully-drive-the-beyerdynamic-t1). I'm sure that colouring the sound using a tube amp would be able to tame the highs and make these headphones truly fantastic but IMO the natural sound of these cans has some serious flaws in the treble. This seems to also be attested by the frequency response curves I've seen for these headphones and also numerous comments I've read stating the same point.
This being said, I really do like these headphones and aggressive use of EQ can tame the treble enough to make it enjoyable for watching movies and listening to music. Without applying any EQ it sounds truly superb for Classical and Jazz music (as long as there is minimal singing IMO). However on the whole, I will probably sell these and settle for something more bass/mid centric such as the HD 600/650 which are widely praised in combination with my current DAC/amp.