I've been giving the M200's a break and listening to the M500's all evening. Using iPod and Intruder. Staging is wide, almost a surround sound effect. Treble is detailed and I hear zero sibilance and don't detect any peaks I heard on open box. This happens to be one of the better treble presentations I've heard. Seriously nice details in the treble. Midrange is to die for. Little more air than the M200 and added mid-bass over the M200's has a nice effect on vocals. Bass is punchy and fast. Doesn't go as low as the M200's but is detected on music with sub-bass notes.
For the more analytical listener, M500's are the ticket. Not that M200's can't be enjoyed by the analytical listener but is slightly warmer sounding. Both are different and sound amazing. If you're worried about fit, the M500's are a no brainer. Both are extremely comfortable though. It took me about an hour to get the fit perfect on the M200 and I've had zero problems since. It took me about an hour to get addicted to the sound of the M500 and I'm hooked. Value on these, I feel I got a steal on the sound from both.