Having upgraded HD650 cable, noted improvement. Others may not.
we tend to inhabit a section where believing isn't proof of reality. with the hd650 you would need to go for an "upgrade" cable that really mess things up a lot before you could actually hear anything. and that is factual measurable repeatable reality.
the process of changing cable and placing the hd650 back on your head can already change the signature by no less than a few DB. http://www.innerfidelity.com/images/SennheiserHD650.pdf
look at the FR lines. in gray are the measurements Tyll did while adjusting the hd650 on the dummy head to get the best seal/position. as you can see +/-5DB on both ends of the frequencies aren't at all impossible.
just listening to music, then changing to another cable I prepared, with the sides of the plugs very obvious and a switch to avoid having to unplug both cables from the amp, it took me close to 25s with my own hd650. how well can you remember the sonic signature after 25second? yeah right!
even if you actually heard a difference and didn't just imagine it, then you have zero credible way to ascertain the differences were from the cable and not from the way you placed the headphone. (maybe one cable is heavy and the hd650 goes 0.3mm lower on your ears? or you simply couldn't consistently put the headphone back at the same place? but as long as the possibility exists, you shouldn't pretend that you know it comes from the cable. it's simple logic when solving a problem.
the way you did it looks like this: there are 3 people in a room, you decided that the mustache one is the criminal, notice a knife on the table and claim "ahah I knew it was the mustache guy!".
you wouldn't do that on a criminal case, plz don't do it for a cable. don't just pick a culprit at random when you have no way to dismiss the other hypothesis.
now open minded me tries to justify sound difference:
let's say one cable is an ok cable, with the right average specs for an headphone cable(99% of headphone cables). if the other one is super long with something like 10 more ohm compared to the first one. that would make the bass go up close to 0.1DB at 80hz!!!!!!!!!!!!!!!!! because of the hd650 impedance response. now let's pretend that the high frequencies gets rolled off by the length of the cable or some weird choice of diameter or not enough insulation, trying to impact inductance/capacitance(the most impressive way to modify this would be to use a design that is wrong for headphone cables, as default cables are usually already made for a given purpose, so clearly not an upgrade). then because I'm in a good mood and I know for having handled one, that ludicrous audio cables do exist, I grant 0.1DB roll off somewhere close to bat frequencies.
creating in that messed up scenario, a 0.2DB tilt from bass to high frequencies. (trying to do the opposite and improve the cable specs would be a lot harder as the margin would have to be much smaller, and so would be the audio changes).
first you should seriously consider the fact that the default hd650 cable is probably the better one of them. secondly, how confident are you that you would be able to spot 0.2DB spread over the entire spectrum, when you can get at the same time up to 5DB difference from placing the headphone back on your head, and with a 30s lapse between the 2 music samples?
oh and I forgot the best for the end, close to 0.3DB in volume level difference from the 10ohm difference in the cable. meaning that overall loudness difference will be bigger than the actual sound differences.
you can keep pretending that your superior ears can track what we can't, but now I've explained how extravagant it makes you look to simply consider that option.
