Why does it take so many hours to brea in arc preamps and amps?

In the final analysis all we can listen to is manipulated power. But it is manipulated power,  be certain of that.
Is it even possible to listen to precisely the same signal every time, even if we wanted to?
Conversely could we possibly hear marginal differences, even  if we wanted to?

"I have yet to hear a tube amp that sounds as good after 5 minutes than it does after 1 hour let alone 4 hours."

agree. Air, bloom and soundstage are MIA until later on, after an hour for sure. This is the sort of thing that apparently separates the mid fi troops from the rest of us.

After my philosophical bs, 
Let me agree re tube amps! 

Its not just ARC equipment. I was told by a modifier who replaces mediocre components with high quality ones, that quite often the best ones take longer to break in. Now, if I could not hear it for myself? I would call it marketing BS. But, what ever it is that takes place to burn in a component? No one has yet to find the science to show what actually takes place. In the mean time, the fact remains that something takes place, and is discernible. I have witnessed to it too many times to doubt is a real phenomena. Its possibly something takes place on the molecular level that no one yet knows how to measure for. When there are electrons, voltage, and current moving through resistors, capacitors, switches... wires, etc? Who knows what changes it causes? Our ears will know its a real phenomena if our system has the resolve and transparency to demonstrate it.  

>> "I have yet to hear a tube amp that sounds as good after 5 minutes than it does after 1 hour let alone 4 hours." <<<

I think you are confusing 'warm up time' with the process of 'burning in - breaking in.'  Warm up time may take for you 5 minutes and is how long it takes for a component to stabilize after turning on.  Burn in time different, and is a transformation that takes place that causes a component to reach its design potential. Its like breaking in a new car engine that can take hundreds of miles.  Its not to be confused with how long it takes an engine to warm up before running.