break-in periods are an excuse for shops to not honor your warranty. a sub will break in over time during normal playing, and once it is broken in you will maybe hear a little deeper louder bass. maybe.....
when i was smokin tdx's monthly, i heard from so many people that the tdx/d9's need to be broken in. my first tdx lasted about 8 months, i went through 4 within the next six months, braking in 3 of them. the last one i just installed and rocked out, lasted until i decided to change my setup and the first one i had i just installed and wanged away, it lasted the longest. with everyday woofers, just install them and rock out. never had any problems not breaking in a woofer.