FWIW I've just plumped for the K612. I would have liked to have bought the X2, but it's £130 more than the K612, which I picked up for £100. I may upgrade to the X2 one day if I feel the need, but right now think the K612 will be perfect.
The DT990 has a better sound stage I believe, but also a v-shaped sound signature with both bass and treble boosted. I've read the latter can be quite fatiguing which put me off. The pro version has a higher clamping force than the premium one, which might be an issue if your ears aren't completely enveloped by the pads. It also further emphasises the bass, which, according to some, can muddy both the bass and mids to a degree.
The AKG does have slightly pronounced bass (in comparison with other AKG models like the K701) and loses a tiny bit of the clarity the more expensive AKG sets have. For me, a slight bass bump was good, especially as it doesn't impact the rest of the sound. Also, the comfort of the K612 is generally agreed as being excellent, which is very important to me. Overall I felt it gave the best bang for my buck!
The advantages the X2 has are a general improvement in sound across all areas, from bass to treble, removable cables and the fact they're much easier to drive.
Er bear in mind though that all the above is in my (extremely unqualified) opinion! The link I posted is still probably your best bet.
