I really wonder how much we could reduce the weight of the he-500, without affecting the sound too much. I'm still going to wait on an he-6 successor or second hand before going to the 560, even though all the reviews are positive, at least not until a price drop. Really more curious how the 400i turns out.
As you hinted, the only way is to redesign them. I haven't read any comparisons of the HE-400i vs. the HE-500s, but I'm really curious. Once I've modded and burned in my new HE-500s (and they seem new, not refurbished, though they could be; have that "new cans" smell ), I'll do some semi-serious comparisons with the HE-560s. Just putting on the 500s to make sure they work OK, they weight was shocking after wearing the 560s. And those velour pads! Brutal, just brutal.