Open vs Closed brings a lot...
The M50 is more forward, and in your face, especially the difference in bass. The treble is hotter as well.
The HD598 is notably midrich, so vocals are very upfront, but asides from that, I didn't get much satsisfaction out of their sound signature. They're natural sounding headphones, but without anything that clearly pulls me in.
However, they are very different, so it might be a wise choice to get the HD598. Their sound signature is inoffensive and well balanced, but nothing that truly will have you jamming in your seat. However, if what you listen to is very mid dependent, like acoustic, you might love them.
I much prefer the Q701 over the HD598 for the same genres, and the Q701 sounds more engaging to me.
Edited by Mad Lust Envy - 5/6/12 at 11:49am