Maybe it's the same deal as with speaker wire. If the wire is too thin for the distance it can make a difference, but once you have "good enough" you can't get any better.

Have to admit, though, I can see a big honkin' power cord making a difference with a big power amp but not with a player.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8