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598s are just 558s with the foam tape on the inside of the vents removed. Unless you're willing to pay like $50 for weird colours, the tape removal, and a higher number, it's just not worth it

This is for the 555 -> 595 but it's the same for the 558
http://mikebeauchamp.com/misc/sennheiser-hd-555-to-hd-595-mod/