I don't know if this is of any help, but I've tried different cables on an HD650 and didn't find it made a particularly substantial difference. Sighted tests are extremely difficult to make reliable, as auditory 'memory' isn't particularly reliable due to both psychological and physiological factors. This is something that makes comparison difficult in general, and is probably one of the major problems to deal with even in blind testing. Proper blind tests use a large number of passes to try to mitigate the problem, and I find blind tests in general are more reliable for this type of comparison. Anyhow, this was simply my impression. I'm no stranger to listening tests, though, and have tried cable comparisons multiple times. Sometimes I've gotten what may have been subtle differences, but it was never enough to really be certain, to the degree for which I'd be confident in the result.
Anyhow, I agree with you about price/performance ratio in a cable upgrade. Unless you're absolutely committed to pouring everything you have into HD650s, I imagine you can use the money towards improving something else. When I look at my stuff, cable upgrades are among the very last things I'd consider spending on versus other things. There is so much out there that, in my opinion and experience, exceeds the impact of a cable that I probably won't reach the point where it becomes the most economically viable upgrade. It sounds like you're finding, more or less, the same as myself.
Therefore, I think it makes sense to simply seek out the most economical repair solution and focus your funds on other gear, if that's even what you want to spend the money on (don't feel like you have to!).
