My finding with NH and HD650 so far. So after I've read some comments today about owners of NH saying they liked it at frst, but decided to return it after further use, I got curious again and this time decided to try tracks I've never heard before or long forgotten. Long story short my conclusion is this. First and foremost it really depends on the track. Some tracks HD650 do better and on others NH do bettdr. But, generally speakinh, if your thing is listening to instruments without much vocals involved, and like brighter tone of music, then HD650 will do.But if you are into vocals as main stage of the track, then NH will be better. And darker, and slower songs will be much better with NH. It has a bit of romance in its tone and suits late night female vocal listening nice. Hence the name "Night" Hawk I'd like to think. It's also richer and lusher sounding than HD650, but HD650 better with bringing out instruments better and sound a bit more real to me. It's also cleaner sounding.
NH will be more forgiving then HD650. Both don't hurt my ears and are smooth enough, but NH ofcourse smooth'er. Musicality wise HD650 good too, but once you start not paying attention to sound only and start doing other stuff associated with music, i.e, like watching music videos which involves visual as well, you will find NH is just very good with flow and rythem and better than HD650 in musicality. it just "flows" in the backround.
So I take it between the two it really depends on what you listen to and what you look for, but in the end my thing is with vocals mostly as taking main stage, so I prefer NH.
Compare to low budget desktop speakers, I can compare the NH to the sound of Klipsch Promedia 2.1 sound that's still one of the best of all time in budget desktop speakers. But NH ofcourse better in all departments compare to Klipsh. It's just has a bit of tube characteristic in its nature. But it's not syrupy, it's a good tube sound done right.
The HD650 more of Bose companion series 5 speakers, cleaner and brighter than klipsch. But I don't think hd650 better than companion 5.
A lot definitely comes down to the recording, the person's ears, and the system. The thing I found with the Nighthawks like what you say is that they flow and don't distract you, I almost think their best best application is actually movie watching and gaming, they completely immerse you and you completely forget about them. I do prefer them for music to the HD 650's, regardless of complexity and genre, though there are some recordings the HD 650 do sound better on. I view the Nighthawk as pretty much an ultimate all-rounder headphone. I do have headphones I like better for music overall than the Nighthawks, though they are all more expensive than the Nighthawks. I actually like an aggressive or brighter sound for music quite a bit some reason but it also has to be musical and engaging, the problem is finding a headphone with those qualities that don't irritate my ears. I do love the luscious musical smooth sound of the Nighthawks, HD 650, LCD-2, etc. The lush musical sound is what I want my main headphone to be as I can enjoy everything with it.