Depends on your chain/source, and how far you intend to travel in this headphone land.
In short, if you intend to use it with your phone, ipod, laptop etc, the 558 will do.
But if you are serious, you will keep the 6xx even after you have found your endgame rig.
dude I just tested hd598 vs hd650 in store and noticed difference in sound quality. and I was playing from iphone without amp. and I am total newbie in headphones.
The hd650 is definitely better in quality, though it depends on what tonality you prefer. The hd558 is closer in signature to the hd600, in the upper midrange and treble. The hd650 reduces the upper mids and treble while boosting the bass resulting in a much more laid back sound.
I think so, my other favorite headphone is more treble and upper mid focused. Great for gaming and soundstage, but it tends to get fatiguing after a while.
I am buying these to be the more relaxing alt warm sounding headphone. I got to test one out for a few days, I've just been waiting for a good sale.