Pair of senn 555s i originally bought but due to the cracks i sent em back and got the 558s for 20 quid more (total 80 now). They are superior in every way. The whole black look which i think beats the grey. They cushions are firmer. The cups are a little smaller i think but they are no way worse. The plastic of it is a much tougher and more durable plastic than the 555s i.e no cracks what so ever after about a years usage now compared to cracks on the 555s after a week or two.
The only niggle for me is that the velvet stuff, when you have dandruff it doesnt half make em scruffy quickly when u itch ur head lol. So yeah i wish they were more dandruff proof lol if u can say. So better way or "a" way to clean them would be nice i have thought about taking the ear cushions off and chucking them in the washing machine but i probs would destroy em lol. Also you cant take the headband bit off which is one of the worst for getting manky.
Quality wise, they sound fantastic. The bass is so much nicer than the 555s were a lot more stronger and so much tighter. The mids and highs are a bit the same if not a little better.
So theres my mini review of my 558s. Lovely pair, cost me the earth like but im hoping they will last me a good 5-10 years.
Edit - One negative tho is that the cable gets very tangled up with my 360 pad cable. I have to detangle them both regularly and its a bit annoying. Would love to braid the cables but duno if possible or how.