Grados are surprisingly good for classical music, mainly due to their midrange. But the 598s are even better with a more accurate soundstage, less peaky treble range (for those contemporary/ post-romantic works which involve a lot of cymbals), and a larger bass presence (pretty much a must for certain music like Bach's).
Both are great cans for the genre though and the Grados, depending on th emodel you get, may be the better value. But the HD598s are just simply a better investment when you take into account comfort (no Grados can compare o.O ) and flexibility (above average in all genres, which is impressive).
Can't agree on the 598's. That was what I read but I found them veiled in comparison to 325is. Just different opinions.