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

You can philosophize all you want, it just comes down to what you hear. I recently bought a tube headphone amp. It sounded nice, but a bit shrill. I started burning it in. Just 10 hours later, it sounded MARKEDLY better. Not subtle.

YMMV with other equipment, but I would ALWAYS break things in, just in case they need it. Some things definitely do!

Regardless of what we say, what we believe, or what we consider to be proven by the science of the day, does not change the truth. Believing by whatever basis however is real for us. In these small cases of what makes this or that sound better to us, is an important human reality. 

I suppose ignorance regarding the fact that interconnects ALL comprise an, "RLC" network, to some degee, is fairly widespread.      The, "C" is for, "CAPACITANCE" and- like any capacitor; the dielectric will take time to stabilize.      The higher the quality of dielectric, the longer the stabilization period.      I’ve always personally held that Dielectric Absorption has much to do with the way capacitors and cables sound.      Especially, given that the better the dielectric (ie: Teflon/Polypropylene/Polyethylene/etc), the lower the absorption figures will be.      https://www.sciencedirect.com/topics/engineering/dielectric-absorption      Of course, there are those that deny that science, with regards to the sound of a system.      https://www.elandcables.com/the-cable-lab/faqs/faq-what-is-capacitance

I suppose ignorance regarding the fact that interconnects ALL comprise an, "RLC" network, to some degee, is fairly widespread.

Dielectric absorption can be real and can also be completely unproven to make any difference at all in a given system, or the effects could be relegated to nano and pico volts of difference. Proving something has a mathematical difference is very very far from calling it perceptible.

Like vibration isolation devices, would be super easy to measure differences in actual reproduced sound, but strangely enough no one ever has.


In the end, if you personally can't hear it, it's worth ZERO dollars. If you can hear it, is it worth any dollars? That's still up to the buyer.


Best,

E

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.