IMO you want the heaviest gauge available for power amps. For electronics, you don't need 10 gauge but you don't want flimsy thin either.

Blah blah blah, it depends on what you are plugging in.

Headphone amp

How could you possibly settle for anything less than this?
http://www.audioadvisor.com/prodinfo.asp?number=KKPK10PAL

An $8.00 cable, bah! With the above cable only the most elite electrons will visit your device and the vast improvement in sound will leave you absolutely drooling.

Dare I ask what happened to the original?

FWIW, I don't think of a headphone amp drawing that much power. Perhaps you could get away with the 4' version that JG linked to.

Yeah, for a headphone amp any old cable will do. I usually like to have thicker gauge cables on my mixers/power camps/computers, but it honestly doesn't matter that much for regular, non tube gear.