I just demo'd all 3 of these at audioconsultants in chicago, as well as the HD 600 using a pretty decent home theatre DAC and amp, as well as without. Tracks I listened to were classical, pop, and rock.
518: The best sounding without an amp. Warm, colored sound. Not that big of a soundstage, but it was hands down the best without an amp. With an amp, it was clearly the worst. Tinny, muddy and muffled sound, a lot less detailed than the 558 and 598.
558: Without an amp: Cold, anemic and flat. Decent soundstage but everything sounded incredibly cold and flat. Detail was ok, but unpleasant to listen to. With an amp, much better. Good detail, even better soundstage, and everything was still crisp but sounded much more life-like than without an amp. Don't think you would be dissapointed with these.
598/600: Putting these together because I couldn't tell the difference, then again I'm not a musician. Without an amp: Even more cold than the 558, almost unlistenable. With an amp: Great great sound. Huge soundstage, great detail, and good bass. Not a ton of bass like a beyer (tried those at guitar center, huge thumps!) but just the right amount to accent the music and keep it dynamic. I was on the fence before but now I'm planning on getting a 598.
Basically, they all have similar sounds, but the higher you go, the more detail and soundstage you will get. Also, the 558/598's are not worth it if you don't have an amp. They sounded much worse than the 518 unamped, but blew the 518's away once powered well. All were very comfortable to wear.