They use the exact same driver as the Premium version and their driver is very similar to the ones used in the DT880 and DT770. People who own all three cans love to post about how different they are but in reality, they sound very similar to each other due to how similar their designs are. The DT990 has slightly more bass and treble than the DT880, which is in comparison far more neutral and flat. The differences are not enormous though.
They can't realistically be compared to the ATH-M50 because they are open headphones, unlike the Audio Techinica, which is closed. The ATH-M50 has more bass but the DT990 actually has some of the best bass amongst open-back headphones - fairly pronounced and impactful while being quite controlled as well. The DT990 has far better imaging and soundstage and it is also more crisp and clear in the high frequencies (a feature that the DT880 and DT770 also have, its the Beyerdynamic trademark orthodynamic-esque speed and crispness that all these models share). Even with all this treble emphasis, the DT990 is very rarely sibilant, whereas the ATH-M50 actually does occasionally venture into sibilance.
I own the DT990 Pro myself and they are my favorite can so far. I like their slightly smiley frequency response curve and they are still flat enough for monitoring use. I love how they are a combination of the amazing refinement and treble details of the DT880 with a slightly darker sound signature due to the boosted bass - you get the best of both worlds in one can. The disadvantage of this situation is that the mids are a little recessed compared to the other frequencies but this is only in comparison with the DT880 - if we are to evaluate the DT990 without comparing it to its brother, the mids are actually still really good.
Finally, the DT990 Pro is priced extremely well. It uses the same drivers as the Premium model and it is just as refined as you'd expect a previous Beyerdynamic flagship / reference headphone to be. This is quite a good bargain at the price that it is going for.
Edited by jupitreas - 10/28/11 at 11:01pm