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

I have no way to prove any of this, but I "believe" that after listening to my speakers for awhile, they sound better after 30-40 minutes or so than they did at the beginning.  Perhaps the drivers become more compliant (?) or warm up a bit.  Whatever it is, the sound seems to fill the room better, the stage opens up a bit more, and the music becomes more involving.  As for break-in of brand new speakers, the crossover certainly may undergo some changes, although one could argue that certain manufacturers do test runs of new units at the factory to rule out any problems before they ship them out.  But as it is with my present system, with the preamp and amp already in "standby" mode, I have to believe that it's probably the speakers that go through a warm-up period.