My original nighthawk is in repair right now, because of one channel of the thick cable cutting out (had to send the whole headphone). Will take a while before they come back. I went to my local store again to listen to the NH carbon and NO. There I purchased the NH carbon. At home I had the impression that it sounded different from the demo model, but I thought it was just another case of placebo. Still interested, a few days later I went back to the store and took my new NH carbon with me to compare to the demo NH carbon they had there.
I have to say that they sounded very noticebly different, which is totally not what I expected. They have essentially the same sound, but the demo NH sounded more open, more clear, with a cleaner bass. Switching between the two, the difference was very noticeble. The person there said that the demo NH already had more than 400 hours on it, and suggested that I let my new NH play over the night. I always was sceptical of burn in, thinking that most of the time, it is simply your brain getting used to the sound signature. However, I now know from my own experience that the NH really need to burn in before they sound at their best. Whether it is the cable, or headphone, or both that needs burn in, I don't know.
But for the people that get a new NH, I recommend that you do listen to Audioquests advice of burning them in, before you make any critical assesments of the headphone. The same probably applies to the Nightowl, though I don't own one, so can't be sure.