As i know there is a rule, source output 1/8 max impendance of the headphone, where FR does not change. Its something different.

My setup:

WIN PC--> Apple Dongle-->Extension Cable--> Cable X or Y or Z--> Sennheiser ie 600

I hear a difference in all 3 cables, and prefer a cheap 16 core silber platted cable.

And for sure its not some placebo, and i hate the fact i hear a difference, but there is one.

Cant say what changes, and the fact companies selling this very expensive cables cant tell me hard facts like measurements too, makes me believe its not this relevant.

So in the future i will keep an eye on this, but never spend huge money on cables.