I cannot speak for the 518/519, 558/559, but the 569 is a closed back with some minor tweaks to the driver to optimize the closed back system. You obviously know that the looks have changed. With the 598, the drivers are the same but I demoed a 599 and it sounded like there was a tad more bass (since the 598 bass was almost nonexistent, it's not a bad thing. Not Thuddy, but a tad louder. Barely noticeable unless you're really listening for it) and I noticed the mids and highs sounded slightly more natural, but that could just be from a better source. From what I heard with my ears, if you have the 5xx don't upgrade. But if you don't have one already, and the one you want is on sale, I think they are nice. I found the 599 more comfortable and seemed higher quality build than the 598, but not enough to justify the price difference (on Amazon currently the 598 is 142 USD and the 599 is ~$250.) So if the prices were the same, or within $15, I would go with the new model. But because of the price difference I'm more interested in the HD 600 (since it's only $15 more than the 599)
Something I find odd is the huge price drops recently in all of the 6xx, 700 and 800's. Makes me think they are soon gonna come out with maybe a new version of the series (say, maybe a 601, 651, 701, 801) being the HD 600 is 265 on Amazon, 650 is around 340, HD 700 around 400, and the 800 at 930 ish.
I would be open to an updated version of them, so long as they keep producing the current series along side the updated versions.
