Help needed!

If the K551 is anything like the K550 that I have then they should sound very good. I'm assuming that the only major differences between them are cosmetic (use the same drivers??). From my experience with the AKG K550, they are a very well balanced headphone with regard to frequency response. I haven't really noticed any emphasis on a particular frequency range with them. For this, I trust them more than the HD600, which has coloured upper bass. Not good for mixing and mastering of music. However, the K550 lacks "slam", in my opinion. In other words, it lacks some dynamic energy so instruments do not have the convincing realism that they should have. They are still great however.