I'm not usually very active on these boards, but I'd like to share my experience/opinion on the HE-6 regarding build quality.
There really seem to be some quality control issues: MY pair did not have the white damping material in one cup, causing the black foam to "fall" around when handling them. Also, the foam flap covering 1/4 of the driver was folded back on itself. After removing the earcup to straighten it, the plastic plate from the earcup snapped when trying to put it back on.
The connectors on the headphone-end of the cable also seem a cause for concern: These connectors are meant to be crimped (clamped with a pair of pliers) onto a coaxial cable, which will give a very strong connection and strain-relief. In the HE-6 however, they are soldered, with the shrinkwrap added for strain relief.
Now I haven't taken them apart, but as far as I can tell, the shrink wrap is only attached to the loose outer ring of the connector (check your spare connectors to see what I mean).
On my pair I could easily twist the shrinkwrap and cable, independantly from the connector, it could even hear the wire in the cable creak when doing so.
As a solution I have added another layer of shrinkwrap, over the entire connector, (not the nut ofcourse), preventing it from twisting, wobbling and creaking.
I did contact Hifiman and they were quick to answer and help me find a solution to my problem with the missing damping material (I used "Twaron" in the end, a new kevlar micro fiber with extremely good damping properties, probably overdamped them a little , but I like the sound), but the cable/connector construction remains a cause for concern, and the Earpad construction is completely unnaceptable on any 150€ + set of headphones in my opinion, they could have at least picked a sturdier type of plastic.
Because of their great sound and comfort (I find them amongst the best sounding, most comfy headphones I've owned) they are certainly a "keeper" for me, but they are not without problems.