I have a set of open-back Sennheiser's, HD570. I tried a few different ones in Toronto when I was at Bay Bloor Radio, I was there for schooling in Toronto. I found the Grado's had too much pressure from the headband and were not comfortable for extended periods. The Sennheiser HD600 was not worth spending the extra dough. I found that the headphone out on my Marantz AVR is inferior to the one on my Akai AM-U7 integrated, so they must be kind of finicky.
I can wear those Sennheiser's for hours. They don't play extremely loud, which is OK. They do have nice bass, but not over-exaggerated.