Wow. Nice response and a lot of good information for me. I just got the 550's and an E7 amp. I'm running mostly off my iMac for now and I find that they sound very different depending upon the music. Almost a completely different sound signature. (Keep in mind, I'm a newbie should I say something ignorant) The E7 makes only a bit of difference in SQ but a huge difference in volume capability so it's cleaner at higher volumes.
I now understand what people were saying about thin sound. But put a little pressure on the cups and it gets rich and full bodied. Despite their fully adjustable cups the 550's are a bit loose which adds to the pressure on my head from the band. Not horrible but you feel it's presence.
Is there a better amp/dac for these that will improve the low end so I don't have to break out my old 1970's Bjorn Borg head band?