I compare it to breaking in a new pair of jeans, as the jeans age they become more pliable and comfortable like the suspention of a subwoofer, the spider softens up over time and provides less resistance to movment increasing sensitivity and deepening the tone slightly.
When I first bought my Eclipse 8712.8's I thought they sounded crappy, they had crappy sensitivity and didn't sound right, so I went back to the retailer and he explained that the suspention needed time to break in and that once it had softened up the 8700's would kick ***, he was absolutely right they sounded like a completely different sub after 3months and to this day I still think those are the nicest subs I ever owned (stolen..sob, sniff..)
Some facts I was told-
Sealed boxes will increase the time it takes to break in the sub as the sealed box limits excursion compared to ported
The time it takes can vary from 1 to 6 months depending on how often you use the subs and what kind of enclosure
You can speed up the process by playing a low wattage tone overnight to the sub.
-play a tone close to the FS of the sub
-play the sub "free air" and ensure that the rear vent is not blocked.
-only 20 to 50 watts will be required for full excursion since the sub is playing with no enclosure.
-check the sub after an hour to make sure its not overheating, warm is normal but hot with the smell of burning glue means you are using more wattage then you need to.
hope that helps