Ok, so I just went to Guitar center and tried out 3 different pairs. All of them were tried directly out of my IPod.
Firstly was the ones I have been thinking about, the DT 770s. The isolation was good, about what I expected. They were pretty comfy, although being closed their is obviously a bit of tightness. Now, the sound. The bass was not at all overpowering as some people describe, I was let down in this regard in terms of bass impact, although I suspect not having an amp is part of this. The DTs were detailed, although compared to my AD900s they sound a bit veiled. Lastly, and somewhat catastrophically, was the treble. As you said Tiemen, the sibilance was pretty bad. Any high pitched "S" sound from the vocals was making me uncomfortable/cringe. I really wanted to buy these, but I just can't justify buying a phone that gets tiring after a few songs.
Second, the salesman somewhat pushed the senn HD280s in my face. I didn't have much expectation from these since I knew they were a cheaper phone, although they have gotten some good reviews. Putting them on, I was shocked to find that people gave them such good reviews - these things were not only damn uncomfortable (especially with glasses on), but they sounded like tin cans. Anemic and harsh, I took them off pretty fast and asked for the last pair I wanted to try.
And that last pair was the Ultrasone HFI 450. The isolation was about 80% or so of that of the DT 770s, which was still acceptable but probably as low as I'd go isolation wise. The bass was about 75% of the DT 770s (in terms of brute force, id say they were similar in tone), which is decent but left me wanting more for heavy hitters such as Massive Attack's "Unfinished Sympathy". They were still a bit veiled compared to my AD900s, similar to the dt 770s, but I suspect any closed phone will not sound as close as my opens do. The treble was not as harsh as the dt 770s. It was a similar overall tone, in that it did not sound terribly warm, but the sibilance was not nearly as pronounced. The real kicker was the price tag, 99$.
Overall, I don't know that I would buy any of these phones (unless I try the DTs with an amp and find the harshness to be reduced), but I must say, the Ultrasone's, for half the cost, were about 90% as good of a headphone; this got me thinking what Ultrasone's that sound 200$ could do - although I'd still be worried about a similarly cold sounding tone.
So, for now, the search continues for a warm sounding, good isolating, bass hitting, closed headphone. (Although, if someone with the DT770s can describe if they find the sibilance to be reduced with an amp id still be very interested)