Originally Posted by The DarkSide
Actually, on head-fi, quite a few who have heard both have stated the S500 is an upgrade. And for about $50 on Amazon you could purchase the S500. That means, even though it's not officially available in the USA, it's still easy to get.
Obviously since it is a more expensive model with a larger driver it is expected that people should prefer the S500. Some did prefer the S400 though. At $50 though the S500 is double the price of the S400. If the S500 isn't that much better though then the S400 is the much better buy.