I have heard the 598s have slightly better sound stage and more rich highs while the 558s have better low end, I personally own the 558s and love them. They put a piece of foam in the inside of the ear cup on the 558s many people claim if you remove this piece of foam (they call it the foam mod you can google it) it opens up the sound stage more and makes them sound near identical to the 598s, however I have never tried the 598s so I cant say for sure.
Youtube review by zeos he talks a bit about the differences in his opinion:
https/www.youtube.com/watch?v=9LWDMzDVIHg
HiFi Hevean review: https
/www.youtube.com/watch?v=P2zlrGrqYL8