That's certainly the popular HF mythology, but many disagree and favor silver for its superior electrical characteristics/conductivity. The whole claim that copper sounds "warmer" or somehow tames the hd800's highs is something that would show up in simple FR tests if it were true, and it simply does not.
As discussed ad nauseam, for those who want to tame trebles and reflections, there is a very simple tweak that does this for real - and for next to no money: the jazz/anax mod. Alternatively, if you throw an impedance adapter between your amp and hd800's, midbass will increase at the cost of damping. (This has been measured and demonstrated - in fact, Senn's own amps have a relatively high balanced output impedance for this very reason, IMO, and not because Senn is too incompetent to design a low output impedance amp.) Know what else is the equivalent of an impedance adapter? A cable with high resistance. Thus, it may well be the case that some hd800 owners prefer poorly made cables over well made cables because the former will actually change FR and increase midbass response.
Another interesting consideration - the commercial cable makers all seem to be using eidolic xlr plugs (frankly, because they look fancy), but neutrik is the only company that makes 4 pin xlr's with pure silver contacts. Those who do use neutrik's generally use the model with the inferior gold plated connectors because they want the aesthetics of an all black barrel/body. The design choices are being made based on fashion, not function.
I've been on these forums a long time, built my fair share of amps, cables, blah blah. Cables are 99% about style points and bragging rights. Unless you already have a summit-fi dac and amp, cables should be the absolute last purchase on anyone's list.