As far as proprietary cables, all audiophile headphones use proprietary cables.

Single crystal copper means that the copper is done in an Ohno Continuous Cast process...whether that matters or not for the signal is another matter, but it is a different type of copper, and it's what cable upgrade enthusiasts go for.

My experience with sennheiser were 2 malfunctioned cords on the PC363D model despite extremely delicate and careful handling in less than 1 year, and speakers frtizzing out on 2 different GSP350 models hardly anythng audiophile cans but enough for me to be extremely cautious of Sennheiser as a company despite there "German Engineering". The ones I tested at my local store the speakers worked fine. I'm not saying there a bad company at all in fact the PC363D's felt very nice before the cables went out, they obviously produce some great cans that a lot of people like them, and I do too -- they're the only ones that seem to fit my ears (after going through all fry's selection) and seem very comfortable stock. I just wanted to add my personal experience and 2 cents with Sennheiser. I've never gotten one of there products from Massdrop yet but many people swear by them. :~)