All the 6x0's have this smoothness to them, in my experience, that you might be hearing as muddiness. Not a lot of things seem to quite pop out like they do on many other headphones (things like the attack of snare drums being the exception in the case of HD600/650s). And they tend to roll off pretty hard at both ends (i.e., not much bass or treble extension).

But they're pleasant sounding headphones that emphasizes the vocals. Dynamics are good when properly amped, and resolution is good though perhaps not as apparent. Ultimately easy to listen to and enjoyable headphones imo.

I don't think they're for everyone. but I like them for what they are.