The HD 555/558 and the 595/598 use the exact same elements inside, only with the 55x being crippled by using a strip on foam inside blocking 50% of the "openness", so I suggest you go for the HD 555 if you can, since they're cheaper than the 558. At least it is around here(~20€ difference)
HD 555 92€
HD 558 112€
HD 595 170€
HD 598 198€
These are all actually the same headphones with different designs. Also note that these cans are not bass-rich. Also, the HD 500 series need proper amplification, your mp3 player won't cut it.
I used lossless audio and a X-Fi Titanium sound card to check the difference between the HD 429 and the HD 595. The 595 really lacked bass in comparison and my phone(Samsung Wave) couldn't amplify them enough. On the other hand the HD 429's were well amplified on the phone and had good bass on the PC.
The real difference was when I started listening to vocals, guitars and live performances - HD 595's were simply stunning - brilliant vocals, great accuracy. HD 429 sounded very bassy and the vocals weren't as nice in comparison.
Edited by Degwees - 5/20/12 at 5:07am