I own both the Senns. HD518 and HD598. The 518's have a bit more bass and lower mid-range than the 598's, so they sound a little bassier than the 598's. The 598's also have a bit more upper mid-range and treble. They sound a little brighter with a more open and airy sound than the 518's.I read the review but let more undecided what headphone to buy. Since I don't live in US my options is very small, Sennheiser is easier to find in budget around 200~250.
Today I have an Audio Techinica ADH-AD700x, its great for gaming but poor for music and movies(almost no bass), so I'm looking for one all around headphone for Gaming 70% and 30% music and movies. I was Intending to buy HD 598 but more bass of HD 599 let me in doubt ('cause AD700x lack of bass) but I'm note sure about bass 'cause a year ago I had HD 518 and I don't like, I think HD 518 has too much bass and affect my gameplay.
As far as bass goes, if the 518's were too bassy for you, the 598's would probably work better for you. However, I've never heard the AD 700x, so I don't know if the 598's will have too much or too little bass for you.