Very similar. If you really want me to split hairs, then I'd say K701 is very slightly more spacious sounding, slightly wider and more expanding soundstage, 990 is a bit warmer overall, with bit more weight in the bass and a more fluid, wet bass, while 701 has a slightly tigher, less impactful and drier bass. Mids are more natural on 990s, things just sound more realistic on then, but 701 has a bit more forward mids. But again, taking 990s off and putting on 701s sometimes makes them sound a bit plasticly and hollow. In terms of resolution, 701s maybe have a slight edge, but details pop out more clearly trough the 990s. One area in which 701s are ahead a bit more is their speed and dynamics, area in which I'd put them above T1s as well, but thats rarely noticeable and difference is small again. Overall, I prefer 990s for vocal, acoustic, live recordings, rock, metal, etc. 701s are better for electronic, orchestral, classical, chamber and similar music. Although I want to repeat, I'm splitting hairs here, they're remarkably similar. If I had either, I wouldnt buy the other as well.
Oh and 990s are better made, much more solid and much more comfortable, typical Beyer, and Akg doesnt have a chance in this area.
I'd just like to add that they are both excellent headphones that need excellent gear to sound the best they can, and this is especially true for the K701's. They're the hardest to drive headphone I have. T1's are a joke compared to them, they sound good even out of an iPhone, but K701's can really stink without a proper amplifier. DT990's like amplification too, and they both like a good source. They can hit way above their price points. There are things I prefer on both DT990s and K701s over the T1's, and none of them are outclassed by T1's out of a $2000 headphone amp and DAC setup. It's hard to choose a winner, it really depends on the music I listen to at the time and the mood I'm in. Technically speaking, I think K701's might be a hair ahead, but overall, DT990s sound that tiny bit more natural and realistic to me, they have a more "3D" soundstage and more coherent sound, like every part fits exactly where it should, with any music I try on them, while K701's can be hit or miss in some cases. Sometimes they can sound mediocre, sometimes they are mind blowing, and they're usually mind blowing with music I'd never assume they work great with, like electronic stuff from Infected Mushroom. On the other hand, I've never heard the guitar at the beginning of "Pink Floyd - Wish you were here" sound as tactile, convincing and real as out of the DT990's...