I would like to emphasize that I respect the fact that others enjoy this headphone. I didn't mean to put people down; only the headphone. I enjoy it too; just only a small percentage of the time. I'm not going to lie about my negative experiences. Much of the time it sounded like the complete opposite of how instruments actually sound. (Not all the time, of course. There were moments when it sounded just fine; usually when the recording was less complex.) I'm not going to be able to express that in a way that doesn't offend anyone. So I apologize for upsetting you, but don't apologize for not liking a headphone as much as others do.
My primary reference is the sound of real instruments. I have been a musician my entire life. Vocals. Piano. Guitar. Trombone. Orchestras. Wind ensembles. Marching bands. Jazz band. Metal band. You get the picture.
Just look at the measurements to see how far away the NightHawk is from neutral.
http://goldenears.net/board/GR_Headphones/5772639
Now contrast that against the STAX SR-207, which follows the diffuse field curve almost perfectly. (Oh, and these measurements are with upgraded pads. With the stock pads, that upper frequency peak isn't there.)
http://cdn.head-fi.org/6/6d/6de6f4f3_STAX_SR-207_EP-507_SB2217.png
Normally this wouldn't have been an issue, because that can be tweaked with EQ...but even after I equalized it every which way, the NightHawk still sounded totally artificial to me most of the time. It has other problems besides frequency response. And I'm not going to deny that it does have good technical performance in most respects, like low distortion, good detail, transients, imaging, etc.
I instantly adapt to headphones. Switching between two headphones within moments or living with a headphone for months: same difference; that is, no difference. (But when I say adapt, I don't mean altering my perception, because it is never altered in my case; I just mean being able to assess sound in general.)
Price is not quality. As I mentioned, despite being technically inferior in just about every way, the KTXPRO1 is still more balanced overall. I am used to the sound signature of real life, and it just so happens that the KTXPRO1 is closer to that overall than the HD 800 and NightHawk, but only when we're talking about the stock tuning, not other technical aspects in which it is easily outperformed. Again, that's just without EQ. You can make nearly any headphone sound much closer to neutral if you spend enough time equalizing it.
I usually want as much accuracy as possible and prefer neutral headphones like the STAX I mentioned above. I hate bright headphones. The brightness of the HD 800 bothers me too. Notice how I frequently talk about EQ. It's because the frequency response issues of headphones can be fixed with EQ, for the most part. And the HD 800 is on a completely different level of technical performance, which is why I said it annihilates the NightHawk. The NightHawk still sounded mostly mediocre to me even after extensive EQ.
Also, I theorize that the brightness of the HD 800 is due to dipped mids more than treble peaks.
http://cdn.head-fi.org/a/a9/a9650bb7_Sennheiser_HD800_Frequency_Response_HRTF.png
(This graph is mislabeled. Just compare the raw measurements to the green line.)
I also play instruments, but our hearing is obviously very different. (Nothing wrong with that.)
I was never aiming for a neutral headphone. I wanted a musical headphone, that can engage me to the music and I can have fun and relax while listening. For me NH fits that bill perfectly.
Instant adaption to new sound signatures is a rare gift I have not heard about yet. Good for you.
I also know, price does not indicate the real value of a headphone. (Or anything.) It was just a bit surprising, that a $30 headphone comes out as a winner compared to a $1000 one.
You did make me curious though about that Koss, so I might buy one just to hear how it sounds.
Differences make this world beautiful. But at least one thing is common here on Head-Fi in everyone, the love of music!