But I will say that Sennheiser is one of my favorite companies. I've purchased 5 different items from them over the past 7 years or so (HD 555, RS 180, MM 550-X, CS 686G, and the GAME ONE headset) and they all still work to this day with fantastic audio quality. I broke the MM 550-X, out of warranty and totally my own fault, and they still replaced them with a brand new set.
Are you saying that the feeling of pressure will stretch the eardrum or something?
I've been told that from an audio standpoint, the only difference between the 558s and the 598s is the bit of foam stuck to the inside of each can on the former. Supposedly removing this foam improves audio quality, although having done this with my home pair but not my work pair, I can't say I have noticed any difference other than perhaps that ambient sound is a little easier to hear without it. I also haven't noticed any difference between my 558s and a pair of 598s I once had the opportunity to try, thanks to a former manager who had a real thing about one-upmanship. So I'm not sure quite what you get for the extra $50 other than style. (Though the 598s are very stylish!)
They're circumaural, so they sit comfortably on the sides of your head rather than mashing your pinnae flat, and their velour pads are nicely breathable; they also mass only a quarter kilogram (260g), and they're an open-back design so that ambient noise is merely reduced rather than totally occluded. (Also, unlike closed-back and especially noise-canceling headphones, they do not produce a constant pressure on your eardrums, which has been suggested to cause hearing loss and certainly plays a role in listening fatigue.) Their reproduction is quite good as well, with a surprisingly flat frequency response for the price range; if that suits you, you're good to go, and if you prefer otherwise, they won't interfere with you EQing your music to your heart's content.
Depending on where you find them, they run anywhere from $100 (Amazon Prime) to $180 (MSRP), so they're considerably less expensive than your QC15s, too. I really can't recommend them favorably enough.