I own the HD 518, HD 558 and HD 598. Which one is better? Well this is entirely subjective. After some research I did confirm that all three have the same drivers, just that the cup arrangements are different. With that said the HD 518 offers incredible value. It gives a well balanced sound signature, however the sound stage is not as good as the higher parts in the former 5XX lineup. The sound has a slight closed feel to it even though this is not a closed pair. Personally, the HD 518 is the under appreciated step child in the line up due to its positioning by Sennhieser itself. The value is incredible for the price I paid for it. It was retailing at around $67 on Amazon when I picked it up mid 2016. That is till the new 500 lineup showed up then it became more scarce. On closer inspection the HD 559 mirrors the technical specifications of the HD 518 exactly. Could the new HD 559 be a dressed up HD 518 or is there actually a difference? Well again it all goes down to what changes happened in the cup. Personally, I would not expect much of a difference.
The HD 558 has got well defined bass which is not boomy by any means and does a decent job of representing the high end of the frequency spectrum if you don't mind the slight compression in the high's within a specific frequency range, which is also called the 'Sennheiser Veil' if I am not mistaken. Personally, I am used to it and it does not bother me too much. The music is punchy but on the darker side compared to the HD 598. The mids are not too aggressive but certainly not lacking. This however, does not have that expansive airy feel the HD 598 offers. The HD 558 is an all rounder with more sound stage than the HD 518 but not by much. Picked the HD 558 up on Ebay for $69 new, around June this year. It's decent for gaming and music alike as it does bring a fair amount of focus to individual sounds.
Finally we come to the HD 598. The former flag ship in the 500 series, I would say has a slightly clinical sound with lots of detail specially in the mids and the high's till you hit the 'veil'. The veil is not as pronounced as in the HD 558. The sound stage is significantly expansive than the lower part numbers in the former 5XX lineup. While I am no fan of boomy bass, the bass can have little more impact to create some excitement as opposed to being analytical. The bass is there and well defined but controlled which does not help with the impact but its still present enough to enjoy your music.The HD 598 are insanely comfortable and very easy on the ears, you can wear them for hours on end without feeling overwhelmed or feeling fatigued. The HD 598 are not the best for gaming in my opinion as they are more focused on the overall expansive sound and the impact of foot steps and gun fire are not particularly highlighted over the other sounds. The HD 518 and HD 558 are very good in the comfort department but the 598's take the cake. To be fair the deficiency in bass impact is not a deal breaker for me and I still enjoy listening to them often. Are the HD 598's worth the $249 I paid back in 2012? I don't think so, since its counterparts in the 500 series offer very good performance for half the price if not lower.
These are my subjective opinions on the former HD 5XX lineup from Sennhieser. I use all three in different scenarios both at home and work. But I make it a point to audition them all on my FIIO E10 plugged into my desktop which gives them a fair bit of amplification when making a comparison. Out of the three I would say the HD 598 benefits the most from amplification as it tends to slightly improve impact. Again these opinions are subjective and anyone is free to disagree with them.