I have been a very big nighthawk fan, but I am leaning in the same direction... My poor nighthawks are at risk of going on the sale forum as I eventually work my way back down to 3 headphones for home listening, one for the office (MDR-1a) and one for the road and points in between (PM-3). Right now my primary 3 for home listening are the ATH-R70x, Nighthawk and 1990, but once I save up for a higher end headphone the 1990 will probably stay and the Nighthawk will probably go. The R70x gets a free pass because it has earned the anti-headphone spot, the one where performance counts, but not as much as light weight and comfort and just a different feeling on the head so that when I get a bit fatigued and I just want something with nice tone and balance and otherwise less critical listening I can have a break. This role used to be the K7XX, but I like the R70x much much better.
With the DT1990 I remember comparing with the Nighthawks while listening to some pop music and the synthesized bell sounds (Hall & Oates, Out of Touch) and vocal reverb and symbols were all much more clearly evident and yet still in good balance IMHO on the 1990. It was weird, and I think someone else has posted this as well, but I also felt like the NIghthawks dug just a little bit deeper in the bass but did not have the punchiness, tightness and clean transition into upper bass and lower mids that the 1990 had. And even though the Nighthawks are lighter than the 1990, I like how the 1990 sits on my head just a little bit better. Overall, it is making a very strong case to stick around for a while.
Oh, and this is very unfair because I am working from longer term memory here, and also biased by the general fit and comfort factor for me, but I would definitely take the 1990 over the HD650/600 for either home listening or studio listening any day, but the value prop of the HD6XX is another story of course, that seems impossible to beat at the moment.