I am not sure if anyone has answered this question. I don't come here everyday and read all the posts.
I did have the opportunity to audition the Nighthawk for about 5 days when it was on a tour. But it has been a few months so I can only comment based on memory.
The Nighthawk is a very warm set of headphones. The soundstage, bass, and mids are their strength. The treble rolls off a little early. The sound signature is similar to HD650 but with more prominent bass impact. The resolution, imaging, and layers were all excellent in my opinion. They are built with high quality materials and are very comfortable to wear. Although they don't isolate at all. The sound would leak like crazy. Also I think they are over-priced.
I do like Nighthawk and I think Nighthawk and TH-x00 are in the same level in terms of sound quality. I only wish if Nighthawks are priced similarly as well.