I don't know why some continue to claim the 598 is "more refined" than the 558. It reminds me of the HD650 owners who claim it is an "upgrade" from the HD600. It's not a step up, it's a step sideways. Same thing here. They are basically the same headphone with a slightly different enclosure. The 558 has a slightly fuller bassier sound and the 598 is a little lighter and airier sounding. Neither has more or less detail, grain, instrument seperation, etc. - things usually attributed to "higher-end" or "more refined" sound.
xkgamez - after you decide which kind of sound you would prefer (more air (598) or more body 558)), I think the only question that remains is whether or not you need an amp/DAC to drive them. The answer to this question depends on your source. If you are using a laptop computer or PC with onboard soundcard, then yes, a DAC and/or amp could give you an improvement in sound quality. On the other hand, if you are listening with an iPod, Sansa Clip, or another high quality music player, then I would say no. These devices (I cant speak for other brands) have DACs built in, and neither the 558 nor 598 need a lot of power to drive.