To my ears it seems that the FX500's reach a little lower than the IE8's, and it's more controlled too. There's less mid bass coloration, which helps it all seem cleaner on the low end.
As was indicated already vocals are very open sounding and clear. I wish the midrange was a bit more forward and possess greater texture if that's the right word. I still prefer the IE8s vocals as of now but that's just personal preference.
I tend to turn the lights off in the night when I listen to my phones. Listening to the FX500s in the dark is like turning the lights back on. I think Duncan made a similar analogy..........