There are a number of mods I've been trying to really smooth out HE560's treble, and it's getting to that point where I'm almost completely satisfied. Maybe I'll write up a HE560 mod compilation a few months down the line.
I dunno, I've listened to HD800 on a number of occasions on different systems, and the bass just never impressed me. The treble was super smooth and textured, the midrange was great, but bass felt cheap in comparison.
People who only listen to jazz and seek ultimate microdetail plankton extraction and whatnot will find HD800s perfect, but I listen to too many genres to care for it exclusively.
HE500's treble is smoother, but also darker (more glossed-over and less textured) than 400i's. 400i still has 400's driver genes, so you'll hear a bit of 400's treble characteristics in 400i, but the local frequency response in the treble region is completely different I felt.