To break in a sub, you not only have to run music into it for several hours, you have to run frequencies and transients with lots of sub-40Hz material.
As to need for break-in, it's the deep subs that have the big half-roll surrounds for long excursions. Why anyone would think they didn't need much break-in is beyond me. When breaking in a full-range speaker it's always the woofers that take the longest.
Subs would take even longer, especially when you add in that only long-excursion 20-40Hz signals are going to move the cone much.
How long have you had the sub, how many hours has it been playing, and how much of this playing time contained an abundance of sub-40Hz input?
Also, how big is your room?
