Silver, copper... it really doesn't matter. 99% of all stock cables are fine and the fancy expensive upgrade cables just look nicer.
There is only one way in designing a cable to get a measurable (existing) sound-change, any thing else is just a placebo, is to solder a resistor to both lines.
Most of the time this is a very bad idea, but if you are aware of the Etymotic line and their 75ohm adapters you should get what I'm saying.
Yes, there are graphs, and it's based on science... while upgrade cables show the same fr-graph, harmonic distortion, same .etc as the stock cables...
Also... did you ever look at the capsule of the HD6XX series and how they connect the cable to the drivers? If you did not yet, give it a try it's delightful.
If you still feel the urge to upgrade your cables, at least directly solder them to the drivers instead of relying on this hilarious tiny copper string.
But chances are... they will measure the same (sound the same). You might still like them a lot more, because your brain wants to hear a difference and fools you into believing in snake oil.
Sacrificing a chicken or drawing a pentagram on the back of the drivers will yield the same results though.