I have experienced a rollercoaster ride with break-in with some cables, none at all that I could detect with others. I have had some components that responded wonderfully (or horribly) with cable changes, others that seemed utterly agnostic to cable changes. One thing I have learned after more than two decades is that there really are very few absolutes with audio. You just have to listen for yourself and make up your own mind. Confirmation bias can work both ways, for what you think you hear or don't hear.
Power Cable Break In - Such a Change!
I installed a new AudioQuest Dragon Source power cord from my Lumin X1 to my Niagara 7000. The power cord from the wall to the Niagara 7000 is also a Dragon but the High Current version. I bought that cord used.
So, when I first started using the new power cord everything sounded great. However, after a couple of days I started hearing a strident sound. Especially in the upper mid/ treble region. The bass was also constricted. I started blaming the sound change on another piece of equipment that was installed concurrently.
Now, I was under the impression that the Dragon power cord with its DBS system required no break in. But I did inquire about it to AudioQuest who responded that it would still need about 150 hours to break in. It's been close to that now and sure enough yesterday I started hearing the glorious sound that I heard from day one with the power cord only perhaps better.
I must say the difference during break in and now is quite remarkable, I don't remember any other power cord going through this amount of dramatic change.
ozzy