For aftermarket cables for the HD800, I have the Draug v2 terminating in 4-pin XLR with 6.35mm adaptor - which I bought - and APureSound v3 cable terminating in 6.35mm - which I received for free as part of the HD800 promo I got. As far as I know, the former is pure copper while the latter is silver plated copper. I only bought the former as I intend to get a balanced amp in the future and the latter wasn't XLR.
I can only tell the difference between them on some of the more "loud" tracks I have - in that scenario, the Draug v2 comes across as slightly less strident than the latter. I must confess that I cannot discern any significant bass differences. Overall, I consider both to suit my tastes better than the stock HD800 cable on my current audio stack.
Let me just state that I am still amused I can hear the difference between cables, even when all existing measurements and theory indicate there shouldn't be any. Before committing to a massive purchase, I would advise people to audition an aftermarket cable with the existing stock HD800 cable. If you can't hear any difference, there's no point to purchasing one.