Don't agree that the HE60 > HE90 in any way. I much, much prefer the HE90. The HE60 sounds like a less veiled version of the HD600 but still pretty dry. The HE90 has a much bigger soundstage, is more resolving, has cleaner and harder hitting bass, etc. Best headphone ever IMO.
This isn't true...I've looked inside both and they're quite different. I have all the measurements down for the HE90 but didn't really bother to do the same for the HE60 because the drivers on them are pretty small. Even the hole shape on the HE90 stators is different and pretty damn clever.