Speaker Break In...? Or listener “Break In”?

Both, actually. And, it can take quite a number of hours dialing in the " sweet spot " in the room: speakers relative to the walls / corners, and the location of the speakers, relative to the listening chair. Not to mention, the acoustics of the room.

This, of course, is a varient to the power cord threads, where many people cannot hear differences. And if you read very carefully, the same " few " continue to show up, and imo, show ignorance in listening ability, and making claims, again, that for the rest of us, the majority, it is " all in our heads ". I simply ignore these folks now.