DT990's are built like tanks, they really are indestructible, they are also ridiculously comfortable with lovely soft velour ear pads. The pads are easily replaceable (you can get them on the rain forest) should they wear out. The only negative is that the cable is fixed but as there is nothing wrong with it, that isn't really an issue. Sound wise, they have a nice full bass for an open backed headphone and quite a decent sized sound stage. The mids are slightly reduced in level (compared with the bass) but they are nicely detailed and vocals sound great. The treble gets a lift from 6khz up to 9khz that some people find fatiguing but it doesn't bother me at all.
My newest headphones are the AKG K702 which are on sale at quite a discount at the moment. They are another circumaural headphone with velour pads and are auto adjusting for fit - they are also extremely comfortable. The cable on these is replaceable but as it is already very high quality there is no point changing it unless you manage to damage it somehow. The pads on these are replaceable, though they are not cheap and are harder to get hold of than those of the Beyers. Soundwise these are very different to the Beyers and the X1. For a start there is a lot less bass, though what is there is extremely well controlled and goes very deep, it's just not as loud as you get with the other two. The mids are amazing, vocals and acoustic instruments are fantastic with these cans. The highs are crystal clear and although there is a small lift around the same point as the Beyers it rolls off earlier, so they sound much smoother. The sound stage with these is huge and the separation between instruments makes it easy to just shut your eyes and imagine you are there. These are definitely not for anyone who likes a lot of electronic music or any genre that relies on oodles of bass. With that said, these have considerably more bass now than when I first got them and from what I hear they will just get better as they are used.
Another option would be the AKG Q701. Almost identical to the K702 (above) but with a slightly altered sound signature that gives a little more bass and lower mid-range at the expense of a slightly more constrained sound stage. These are currently selling for about £40 more than the K702's but the sound signature might be exactly what you are looking for, only you can say. There is one major difference between these and the K702's - the headband. For some inexplicable reason, the designers fitted these with a number of bumps on the underside of the headband and depending on how much hair you have, the experience can vary from perfectly comfortable to downright painful. Basically if you have a thick head of hair you'll be fine but if you don't I would avoid. The K702 has a flat leather headband which is very comfortable irrespective of how much hair you have.
The Philips Fidelio X1 is a stunning looking headphone and is extremely comfortable too from what I hear. It's cable, which looks great will need changing as it has a relatively high resistance that can affect the sound quality. The velour ear pads are reputedly just as comfortable as the Beyers but the difference here is that they are not replaceable. Soundwise the X1 has a somewhat boosted bass and the frequency response curve just slopes gently down all the way to the end. Some people find this to be relaxing and others find it dull, I guess the only way to decide would be to listen to some. As you have just bought the STX, I need to say that the X1 is not a perfect match due to it's low impedance. However, it is not overly sensitive to low damping factor, while something like the Sennheiser HD598 is (very). The ideal is to have a damping factor of more than 8:1, with the first figure being the input impedance of the headphones and the second being the output impedance of the amp. In the case of the STX and X1, that would be 30:10 or 3:1, which is a lot less than the ideal. However, because they are not overly sensitive to this, the worst that could happen is that the bass will not sound quite as tight as it could do, though they will still sound fine.
