Can an unused cable break in?

It’s an interesting prospect.

The capacity of Humans for self-deception is apparently unlimited - Mr.Spock the Vulcan

Connecting to the AC Mains Supply Connect the socket on the AC Power cable to the AC Power inlet d on the rear panel of the CD t. Now connect the cable to a suitable AC power point. 

There is no mains power switch on the CD t. To switch off the mains supply to the CD t remove the plug from the wall outlet. For this reason, ensure that the plug is easily accessible when installing the unit

https://www.cyrusaudio.com/wp-content/uploads/2018/10/Cyrus-CD-t-handbook-en.pdf    


For burning in a power cord it will take for ever if the connected load is a CD transport.  It could take many months. It will burn in a lot faster if it is connected to a load like a dehumidifier or refrigerator. ( Make sure the ampacity rating of the cord is equivalent to 14 gauge wire.)  

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Imagination is a wonderful thing!

I agree with Millercarbon's comment about 'burning' in. In my 12 years or so of playing I have always noticed (almost always) that changing gear, cords, interconnects, etc. needs to settle in for several hours if not days to calm or whatever you want to call it. 

I call it settling in, so as to distinguish it from burning in, warming up, and reaching saturation or equilibrium.

Burning in happens only once, when something is brand new. 

Settling in happens every time after something like a cable is disturbed. The more it is bent, twisted, banged around and exposed to temperature extremes the greater the difference and the longer settling in can take. This is why a cable used even for years can sound awful after shipping. Discovered quite by accident when two identical power cords were swapped. Each time the one going in sounded worse than the one coming out- but only for some minutes, after which it sounded the same. In this case settling in took only a few minutes. After shipping it could be hours.  

Warming up happens every time after something is turned off. How long it takes to warm up is totally arbitrary, because it is pretty much open-ended. The longer it is left on and running the closer it gets to saturation or equilibrium, the point at which it is pretty much done changing.  

The reason for "pretty much" is this phase lasts for hours, long enough for sound quality changes to start being affected by AC power and other daily noise cycles. 

Not that everyone will hear or notice all this. But to the extent you are at least aware this is what is going on this awareness greatly improves your odds of being able to hear it when you come across it.