you don't master in headphones, you master on high quality reference monitors. headphones are more or less incapable of accurately representing an audio signal, which is the first requirement for mastering. headphones should be avoided for accurate mixing and mastering tasks in general. i use grado headphones when i'm producing at night so as not to disturb my girlfriend, but i wait till daylight to check my mix and certainly to master so i can use my monitors (mackie hr824s), and even then i feel like they're not up to the level of quality i need! just can't afford anything better...
