The sound of the Nighthawks is kind of hard to pin down because it's a quite different approach than other phones. To simplify, I'd say the NH is very coherent in all respects with nothing sticking out more than the rest tonally or spatially. Where the 650's tend to bring the midrange forward, the NH is very even across the frequencies. Also, the soundstage for the 650's seems wider, but with more of a left/right/center feeling where the NH is continuous left to right with less spotlighting of certain sounds, less pinpoint. I'd say the NH has more extension at the frequency extremes but the bass is more impactful on the 650. All that being said, that certain special magic of the 650's that we all know is still very appealing.
I have to agree with you on that. I have similar impressions of the Nighthawk vs my HD 650 though I do find the Nighthawks a bit wider soundstage wise, though that could simply be differences with my system, but definitely agree about the 3 blob soundstage(left/center/right) of the HD 650 compared to the Nighthawks. With the Nighthawks there is no blobbing of the soundstage like many other headphones as the soundstage is a coherent whole. I personally prefer the Nighthawks as I find them more resolving, refined, and tonally correct, but decided to try the HD 650 more after tinkering with the tubes on my system, was quite a nice experience as it lightly colors the music in a euphoric sense with it's subtle layer of warmth over the whole sound, will spend more time with the HD 650s after spending a month pretty much exclusively with the nighthawks, thought I am currently letting a friend borrow the 650s.