Does Power Cord Require Burn-In To Sound Good?

It could be that you just didn’t spend enough money for the placebo effect to kick in. 

There is no sound difference from power cords other than what you imagine you are hearing. Signal cords yes due to capacitance. Power conditioners help isolate noise but most pwer supplies do that anyway.

Invest your money on better amps or speakers or DACs. Buy a good quality 12 AWG IEC power cord from Amazon for $12.

PC in no way is going to improve anything over the stock cords. Freaking ridiculous to think otherwise. But knock yourself out!!

Anyone, genuinely interested in some of the parameters/considerations, possibly governing the faithful propagation of our signals/voltages, as regards cabling and components, should study the following.      Having no formal (or other, it seems) education in Physics; the typical Naysayer Doctrine adherents, will undoubtedly offer their ubiquitous, Newtonian (1800’s Electrical Theory), ceremonial whines, for your Communion and salvation.               It’s common knowledge, to anyone with a background in the Physics of dielectrics, that the better a dielectric (ie: Teflon, polypropylene, etc), the longer it takes to polarize, with smaller signals/voltages.       https://en.wikipedia.org/wiki/Dielectric      and: https://en.wikipedia.org/wiki/Speed_of_electricity#:~:text=Speed%20of%20electromagnetic%20waves%20in...                  Even the Wiki-scientists are more knowledgeable, regarding modern Electrical Theory (not to mention QED), than the Naysayers.

"Marcomarmo",- said;
"There isn't any physical process that causes any modifications to the marcroscopic or microscopic structure of a cable during what you call 'burn-in' ".

That would be incorrect. 
   And the, "physical processes" involved, never cease.
Imagine yourself sitting for a long period. Not moving. 
Ten feet away no one looking may "see", you move. But you are moving.
You breathe. And the air you breath? It moves too. A truck passes outside billowing smoke. Some time later you smell that smoke. You cough.
And now anyone even 100 feet away, could see you move. 
And the reflected wave of light right now allowing you to peruse this forum? 
Like everything else? It cannot stop it's movement. 
Slower than molasses the "light wave", "Physically" moves. Reactive, proactive and subjectively moving according to the rules. And changing, always evolving. And at some point it shall discorporate from being that "wave".  But It's sum of different parts still exists. And is even still roughly in the same location.
And the "power", in your cable is made up of the exact same articles as the light-wave. Maybe arranged just a little differently, at a level "intangible", to us. And still  with the exact same parts. 
 You cannot, "See" magnetic polarities. But you can "feel" them when strong enough, against your own polarity.
You also cannot see the tree that fell in the forest. But it is still on the ground. 

+1 for "Shunyata Research"! Power and also USB cabling which I secured from them are very good. I personally, have never noticed "much" as far as "Burn-in" with power cables. Maybe that's me, maybe my gear. I DO notice with certain cabling the effects of "Cryo-Treatment" more than "Burn-in" though. I've been doing that myself at times for over forty years. "I started with my guitar strings and hand-wound, (pick-ups), actually". (Circa 1977).
After hearing "Eddie V.H.", loved the way it changed his sound..
"R.I.P." Eddie........