Physical explanation of amp's break in?

"Just curious, is there ANY component in the amp's circuitry that known to cause such a behaviour?"

Nope.

If I agree with Bomarc, would anyone be surprised?

My understanding of break in (of components and cables) is that current flowing in new circuitry is finding the path of least resistance through the lattice of molecules that make up conductors.
For components the highest concentration of conducting material is usually in the transformer(s) of the power supply.
Complete break in takes much longer than most manufacturers will admit. Typically, there is a noticeable improvement in the initial hours of operation with gradual further improvement. But in my experience full break in can take six months or longer.

Resistors shift value, diodes will alter their point of conduction, transistors will alter their point of conduction and their gain curves, the dielectric in capacitors are forming, etc... All of these things do this on an individual basis on their own schedule and the differences between new and "broken in" might seem quite small on their own. When all of these "shifts" are added up and combined though, both the measured and sonic results can be quite different for the entire circuit.

Just bare in mind that stressing any given component, whether it be due to amplitude of signal, thermal changes, etc.. will cause further shifting of value on some components. This can have a domino effect as the newer shifts in value can cause other circuitry to respond differently. As such, it is possible for a device to change even further ( measurably and sonically ) after you think that it is "fully broken in". This is especially true if you start running it harder or in a very different manner than what you used to.

In extreme cases, these "value shifts" for individual parts can be significant enough to cause the unit to operate erratically or not at all. The fact that components do shift in value is what causes them to "wear out" and need repair. I have even seen where a 6" piece of wire inside of a unit developed a resistance of appr 50 ohms. While i could not explain how this happened, it did. To be quite honest, this threw me for a loop for quite a bit of time too. The last thing that one would ever expect would be resistive wire in a low voltage / low current circuit, yet stuff like this happens.

Other than that, anybody that tells you that components don't break in or "settle" is either uneducated about the subject at hand and throwing out a guess at your expense ( IF you believe them ) or knows better and is blatantly lying to you. When it comes to subjects like this, you either know what you are talking about based on book smarts and education and / or first hand experience or you are talking out of the top of your hat. Given the fact that i've been making money by troubleshooting / repairing / modifying / designing electronic circuitry for 25 years now should give you some idea of where i'm coming from in terms of experience. Sean
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PS... I'm sorry if this comes across as "confrontational" to some, but when you try to pass off bullshit or a personal opinion that can't be verified in any way, shape or form as fact, i have little room for compassion.