Speaker Break-In - What Physically Changes During Break-In To Enable Better Sound?

Keith, you've got a severe break in logic at work there. You're assuming "breaking in" is degradation. Everybody knows an engine requires a breaking in for parts to seat and wear property if undue degradation is to be prevented. Practically every complex mechanical device is like this. 

I’m inclined to believe that the thing that’s changing is ourselves i.e. we are becoming accustomed to the sound of a new and unfamiliar device, rather than anything "burning in", or whatever.

I knew someone would say that. I just knew it!

@french_fries 

that's more likely your electronics warming up after this 20/30 minutes period and it's actually quite normal. 

@marqmike 

it sure does sound better after warming up... some systems take even more than that to reach optimal working temperature.