Okay, I am trying this real quick. On HD600s and HD500s.
Immeadeatly I notice the soundstage shrinks, a ton. Female vocals sound slightly more nasal, bass seems a bit less prominent, electric guitar seems more prominent, almost too prominent, also, a flangey sound that colors the midrange a bit (If you can't hear it with the headphones on, try putting your hands over your ears with the headphones off.
(Woah, these sound a lot worse now that I have been using the HD600s a couple days. And to think they used to sound crystal clear to me- damn Sony V200s killed my hearing I guess)
Female vocals sound even more nasal compared to HD600 when closed, but when open the sound, umm, fat? Thats about the best word I can manage for em. Electric guitar - againm, becomes more prominent, the flangey sound I mentioned with the HD600s is even mroe present with the 500s.
Why someone would want to close their open 'phones is beyond me. I might just try and open the V200s up for the hell of it.