I'm happy to say I just picked up the Beyerdynamic T90's for my Crack, and man, they sing. I've been more of a Sennheiser guy, listening to most of my music on my HD650's at home and the Momentums on the go. I was never quite satisfied with the 650's. None of that "veil" nonsense, I just never really dug the rolled-off sub bass, and there was a certain sweetness in the presence region missing from these phones for me.
After reading many a review online for the various Beyer's I decided to pull the trigger on the T90s for just $409.00 on ebay; man, am I glad I did! The bass gave me what I felt was missing with the Senns: greater impact and depth. The midrange is silky smooth and ultra detailed Voices and instruments sound rich and natural with no stridency to speak of.
The treble is just fantastic. This is the biggest difference between the T90's and the 650's imo. They way they've sculpted the treble, it seems to have an emphasis in the upper region: plenty of treble "air", but without harshness in the lower treble. There's a tiny bit of sibilance here and there. For instance, on U2's Achtung Baby, Bono's voice can be pretty raw at times because that's the way it was recorded and mixed. Very little sibilance on the dozens of albums I've demoed. From Master of Puppets to Taylor Dayne to Rush's Moving Pictures, everything is just smooth, detailed and airy. Beyerdynamic has earned a new customer with these superb 'phones. I've done extensive listens with the HD700's and the LCD-2's recently, and I have to say the T90's are not just the best value I've heard under $1000 bucks, but just simply one of the best headphones under $1000, period. Highly recommended as a virtually perfect match with the Crack (mine is all stock in case you were wondering. No speedball...yet).
On a side note, I recently replaced the stock vintage tube in my Crack with a Genalex Gold Lion 12au7...Big mistake! And yes the Genalex was burned in for about 50-60 hours. Where the T90's sounded perfectly even and smooth before, they now sounded overly bright, slightly grainy in the highs, with less bass. I just simply didn't enjoy them as much as with the original input tube. I guess there's something to be said for the quality of those old NOS tubes after all...