$15k/$22.5k cables/amps proportion is not what I had in mind.
In any case, as I suggested above, one must be able to tell the difference between 2' and longer cables. And the difference should be real not something bordering non-existant. It will also depend on speakers. There is no general answer. If someone is concerned with resale, yeah, not shorter than 6', if not - listen. And if auditioning is not possible and there is no-one to advise in the paricular case - 6' is probably most reasonable. Personally, I never really think of the resale value. I upgrade rarely and if after ten years I'll have to throw away an amp I paid $5k for - so be it, that's less than $50 a month. I think about performance and try to get the most of it for the least money, after that business is over and listening to music is in order.