Hi, the self proclaimed DT880/DT990/DT770 series guru chiming in!! Kidding of course, but I have heard every version of every phone int he current production line.
The DT990/600 and DT880/600 sound similar, the main difference is that the DT990/600 has more treble and bass with a bigger soundstage.
The DT770/600 is much smoother sounding than it's 250Ohm counterpart. For one it's much more balanced sounding as I found the 250Ohm one to have a very treble oriented sound with sucked out mids and shallow, but great bass. The 600Ohm version has exceptionally smooth treble that's different than the other variants. The midrange has the most improvement and is full and more fleshed out. The bass is also much better with more body and impact.
I would say the 600Ohm DT770 is perfect for monitoring with it's more neutral and balanced presentation over the DT880/DT990 (but the DT880 is still pretty neutral). The DT880 is like a DT770 but has more bass and more treble, but the treble is not harsh at all.
The 32Ohm versions of all of them sound more like their 600Ohm counterparts, don't misunderstand this as me saying they sound the same, but rather their sonic signature reminds me of them. I found the 32Ohm versions to have a slightly slower sound with more midrange and bass over the 250's, but they have a far less transparent sound and slightly smaller soundstage.
At least the DT770/600, and 32Ohm version impressions were from memory, but I have a 600Ohm DT880 and 600Ohm DT990 on hand so if you need more info on those let me know.
Edited by Zombie_X - 9/15/10 at 6:33pm