558 is similar to the 558 but with a little more bass, little less soundstage and little less quality. the 518 is similar to that but with a bit less bass/soundstage/quality from the 558.
i havent heard about the 559. guessing the 559 is a new version of the 558 and the 599 is a new version of the 598. i can not say how they really compare as i have not read much on them though a few said they sounded a bit cleaner (whether that is a paid positive review, fanboyism or accurate i can not tell you) and they have a removable cable.
given how the 558 and 598 are cheap (at least in usa, not sure about in europe) i'd say they are the better deal than anything new.
i do know beyer cans are cheaper than senn in europe.