I have a DT1990, HD650, HD600, K712 and a few other headphones and earphones. I listen to the DT1990 with the analytical pads, the pads with fewer holes in them. I have had the DT1990 for a few months and the HD650 for years.
Overall, to my ears the DT1990 needs something like -3dB starting at about 8 or 9kHz and going up, and it could use about +3dB at about 80Hz moving down (I like bass when it is well done). This was close to my initial impression and this still holds for me.
Relative to the HD600 and the HD650, the DT1990 mids seem to be recessed but not to the point that this is annoying. Certain recordings, can however, sound a bit confused in the DT1990 due to the treble begin very much in front of the mids at certain points where perhaps this should not be obviously so.
The HD600 is smoother and has a much more natural treble. The DT1990 is a bit brighter but I can't say that its more detailed than the HD600. The HD600 treble is pretty detailed and well executed to me. I would not say the HD600 lack detail overall. Also, the mids seem more detailed in the HD600 and HD650. Right now, the bass in the DT1990 seems cleaner to me than in the HD650 bass in particular on some funk tracks I like. The DT1990 bass is fast, punchy, and well executed.
The soundstage in the DT1990 is wider than the the HD600 but its not nearly as wide as it is in the K712. The DT1990 images well but I perceive the HD650 to be more focused and precise in the center image when I listen to vocals on certain tracks. I am not sure if this is related to the DT1990 treble or not. The very wide K712 soundstage is a bit distracting to me at times where the K712 width comes off on certain recordings as being panned too far right or left if that makes any sense. This is certainly fun, but I like the DT1990 more in this respect, at least right now. The DT1990 is nicely wider than the HD600 and HD650. The DT1990 seems deeper as well, but I have not really figured this out yet. I'm not sure about taller yet either. I have some Chesky recordings coming to me that will help to figure out the height.
Personally, I would not give up an HD650 or an HD600 for a DT1990; however, I would not sell my DT1990 right now either. They are simply different headphones and it depends on what you are looking for.
For me the DT1990 delivers a balanced sound with a great bass (the main reason I purchased the DT1990), a wide soundstage, clear treble and good (but slightly recessed) mids with more energy in the treble than the HD600. The DT1990 with the analytical pads are good. If you can EQ them, the DT1990 are better.