A lot of unnecessary bitching and moaning in here. The OP wants facts, point blank. Opinions don't count at all. A lot of unnecessary chatter from people who may or may not know what they are talking about, but your "break in" could be simply a result of other variables that your complete clueless to because you believe the subwoofer is "broken in".
Honestly though to debunk this myth, would take lots and lots of testing with many different woofers, multiples of the same woofers, different temperatures, different atmospheric pressures, ect.