The HD668 and HD428s aren't really for everybody. The sound is decidedly lean, and while that fairs well for clarity, beginners may feel underwhelmed by the lack of body. You say you liked the XB500s, so if you think the bass wasn't overbearing, you would do well to cross the above two headphones off your list. The K518DJ are a bit more substantial in the low end, like the XB500s but still not as much bass. I personally didn't like them, but I know many who do. Are you open to any other recommendations for other headphones, or are you limited to the following? Have you read ljokerl's portable shootout yet? What is your budget? What do you listen to/out of? What do you look for in terms of sound signatures?