I own all three hd518, 558, 598(Ivory). Pedro, I agree with your assessment of the 558 compared to the 598. The 598's seem to have a slightly brighter quality to the vocals, making them a bit more clear(just a bit). The 558's seem to have a bit more punch, or, a little extra power to the sound,as you say. To my ears, the 558's have a touch more bass "oomph"
Comparing the 518's to the 558's, I would say, the same difference as between the 558's and the 598's; a bit more bass "oomph" and a bit less brightness or sharpness to the vocals.
Generally speaking, I could say, the sound becomes thicker and punchier as you go from the 598's to the 558's to the 518's. Keep in mind though,that to my ears, the differences aren't huge. All 3 sound very similar. It's like the same headphone only tweaked slightly differently.
As far as Z,s review, of the 518 ; I don't really understand what he means by, the 518's are the most neutral sounding. And I don't agree that the 558's are the most v-shaped. I don't hear any of them sounding v-shaped.