People say that the HD280's take extra long to burn in. I think this tends to be true. When I first got them I listened to some Louis Armstrong and high horn notes would distort. Very grating sound. After about 3 days, I noticed these notes were smoothed out and were not distorting. After about 100 hours they sounded even warmer.
Bass got noticeably deeper when I checked at about 15 hours. After 100 hours bass had more presence and was more precise.
Some people say you should burn headphones in using all the frequencies and they use white or pink noise or a mistuned fm station. Some say you should use bass heavy music since you are just breaking in the moving parts in the drivers, so you should try to flex them as much as possible. I just used music of different styles that I would listen to. My stereo allows for 3 cds at a time and I think I used the Red Hot Chili Peppers for heavy bass/fast tempo, Duke Ellington for strings and horns and some alt-country CD that had a good thick stand-up bass sound. I just left the stereo playing for a few days and would check the sound every so often. (Cool thing about the 280's if you place the ear cushions together, you won't even hear that the stereo is on.) Every so often I would put a different CD in, but I always had either RCHP or Parliament in for real heavy bass. Put the volume at moderately loud. Loud enough to flex the drivers, but not loud enough to break your new headphones. I don't know whether my choice of music had any effect on the eventual sound, but the burn-in effect is quite noticeable with the 280's.
I find the headphones to be very revealing. I have definitely heard things that were left hidden by other headphones. I don't have too much experience with high-end stuff. But I think they may be even more revealing that my grado r60s, this may be due to the excellent isolation.
I am listening to the beginning of Quadrophenia with them right now and the ocean sounds have never sounded this good.