I recently upgraded from 5pros to ie8, and also have HD555.
At first, ie8 seems very muddy compared to 5pros (not to mention HD555). The bass is overwhelming and uncontrolled. After several hours (+-20), things get way better.
Bass-wise, ie8 has more quantity but it seems slow in comparison (currently, +-50 hours of burn-in, hope it will get tighter). There is a big midbass hump, that covers low bass, even more than on 5 pro. You like it or you hate it. Personnaly...well, I think I hate it. HOWEVER, the midrange gets better and better as time goes, and is very warm as well as detailed. 5pro mids seem thin in comparison. Highs are very natural, way better than 5pro, imo, but are a bit shy at the moment. But it just could be that 5pro's highs are emphasized around 10-12khz. IE 8 seems to extend further.
Soundstage is nothing to rave about at the moment. I must say that I am a bit disappointed. Maybe I expected too much? I mean, it is not bad (I think 5pros are good also) but when I listen to music, it is not the first thing that strikes me. HOWEVER, it is very recording-dependant.
So, that are my impressions
but maybe you should hear someone's who listenened to a fully burned-in, 400+ hours ie8.