You might notice some changes in the acoustics. Because the voice coils are loosing up. I noticed that after about a week of breaking in my 10"'s at a moderate volume the amp didn't clip at hits that it normally did when i first got them and blasted them right out of the box irresponsibly. You'll probally notice your subs getting louder at the same volume levels as well and even playing at better sound qualitity.
you should break in a sub at half its rated power or maybe less to loosen up the spiders and break in the entire motor assembly...
if you do this and your sub has a problem, then you can take it back right away knowing you didnt blow the sub yourself by blasting it with full power and tearing a spider or burning VC's or anything like that