What is "break in" and what difference does it make? In amps? Preamps? Speakers? More?

instead of calling it "break in" we should really call it "interval of stability". 
from the time an electronic component is made until the time it eventually goes out of service every single part in that component is undergoing physical and chemical changes.   
"interval of stability" is the time it takes for a newly made electronic component to reach a point in its life where it has reached a relative stability, that is where the rate of change has slowed to the point where it has become relatively stable.  

It should be mentioned that cables and electronics never completely break in as much as we would like to think so. The reason I say that is even when heroic means such as the expensive Cable Cooker are employed to break in cables the process should be repeated every year or whatever as the cables tend to revert back 🔙 to their former state over time. The music signal itself is not sufficient to do the job completely or to maintain that level of performance. By deduction, the lower signal cables and wires in the components behave similarly, and are probably even more resistant to complete and thorough break in.

Read my "Audiophile Law: Thou Shalt Not Overemphasize Burn In" at Dagogo.com

I am not interested in debating my findings. 

Why not a mixed of the two? In real life all objective perceived changes are evaluated by a subjectivity...Why it must be objective pure changes or subjective pure changes? Why one must negate the other?

Reality is complex precisely because it is something encompassing what we name objective or subjective....An electronic complex materials can look for his relatively stable working platform in time, (break-in) and we all biologically and psychically adapt ourselves to a new phenomenon and integrate it by transforming it ( another break-in)...Just the words of an ignorant poet tough... :)

Right. Evidence suggests the universe itself is like this right down to the smallest details. Even when we say be objective, the root word object is a thing. But then we look closer, there are no things*. There are only processes. We think the things are matter. Solid. Objects. But the supposedly solid objects are all made of electrons whizzing around. Then it turns out not even the electrons are things, they are not objects, that is just how we think of them. Quantum physics however proves the electron is not a thing in one place moving around it is instead a cloud of probabilities. The objective literally is subjective. And, since the cloud nevertheless makes the object, vice versa.

* The actual quote is There are no such things as things. https://www.youtube.com/watch?v=IGhDkh972Zg