I've owned Hifiman HE6s and now have owned HE500s for a while and I don't perceive any problem in the build quality and they have presented no problems at all.
I've even gotten used to changing the cables and working with the small connectors and have done this a few times and they stay on tight and secure.
+1 i agree completely! Never had problems with all hifiman's i owned incl. the hm602 dap