I just went into a spending spree moment and got a pair of HE-6s. I may not have enough power (SPL Phonitor) to drive these hungry beasts effectively but will try to do a comparo between them when I get back in a couple of weeks. Will also try to borrow the HD800s from a friend to get a comparo across the upper end of Senn's dynamic can range.
Will be an interesting & fun end of Feb/Mar! :D
Can't wait to hear your thoughts, also on the comparison between HE-400 and HE-6--