Summary: The sound of the MS400 is marginally better than the MS300, with better bass and a less colored midrange, but the highs are still softer than headphones like the v-moda M80 and Senn Momentum, which are already soft by hi-fi standards. Detail is a problem for the M400 as much as the M300, and at the regular price, is not a good value based on the sound.
Summary: About me: Newbie audiophile at best, I've been rocking a set of Senn HD280s in silver for probably 7 years. I have never had an issue with build quality or sound quality, and have loved them since I got them. At the time, they were a good bang for the buck, and I still think that they sound great (at least my pair does!). I run sound from a MacBook Pro, and iPod Touch (gen1), and iPad (gen1) or my LG P999 through any number of players.
Pros: Look great
Cons: small cans, claustrophbic sound, muddy and unclear.