I have HD558's, and have tried HD598's. As I said in my review which you can find here on head-fi, I challenge anyone to find me sonic differences between HD558 de-foamed and HD598. They sound almost exactly the same when HD558 is modded (and the mod takes 2 minutes, its reversible, and there's no risk of damaging anything). Differences between 558 and 598 are there only initially due to stronger clamp force of the headband on HD598, which evens out after time. IMHO, with 558's and 598's, its about the looks. Which ones you like more. I liked HD558's more, because I thought 598's might get dirty.
Also, to answer the question about other headphones in that price range.... there are no others which are as good as HD5X9 series, from my experiance. I tried every single popular headphone from 150-250$ when buying these, and HD558/HD598 came as a clear winner. Best sound balance, best soundstage, best comfort, no audible problem with any part of frequency, easy to run, excellent detail, very little headphones can reveal detail as effortlessly as these without being bright...akg k701's come to mind. To me, they're a perfect balance between warmth and brightness (no seenheiser veil with hd5x9 series), without sounding dull, at least after burn in.
However, even though they get very loud from any source, if you read my review, you will see that they DO need amping to show full potential, otherwise they tend to be bass light (there's a huge spike in "amp hunger" from 40 to 150 hz frequency range), and lack some fun factor or snappiness.
Also, they do need burn in, although, as I said, I think it comes more to headband and ear cups "burn-in".
but don't the 558's crack? if you de-foam them, you're voiding the warranty and having them crack after that would be terrible. From what I've gathered, they have a high probability of cracking.
Edited by DreamKing - 10/11/11 at 8:38am