I just obtained new units of both. The MX450 sounds initially more attractive, but with time it is apparent that it has a terrible coloration in the upper midrange and a lower treble peak. Nothing sounds right, and no EQ setting on the iPod can possibly compensate.
The MX400 sounds dark on initial hearing, but that darkness is due to a rolloff of the treble that is smooth. Settings on the iPod such as Rock compensate for this nicely (Latin and Classical are almost as good, and Electronic is fair). Once EQed, the sound is smooth, extended, fairly articulate, low in grain, and very natural with all types of music. Highly recommended for the price, assuming you have the ability to use EQ. Probably a bit too dark for the Shuffle.
The MX550 is said to be just the 450 with a volume control (not that this won't affect the sound). Same for the 500 relative to the 400. Not sure this is true, but since the price points are so close, I tend to believe it. Volume controls tend to do something bad to the sound, being cheap, so I would avoid them. AFAIK, there is no other difference, although some people will hear differences due to the volume control's insideous effects and unit-to-unit variation.
