Why does my subwoofer sound like this?

[CXK]

I've had this Definitive Supercube I for about 11 years now. About 3 years ago it started making this "rattle" sound. It's not constant, it seems to only happen when its playing bass sounds, its far more noticeable on discrete bass as opposed to bigger booming sound effects.

The sound that's playing in this video is a heartbeat sound from Godzilla Vs. King Kong. I asked Definitive if its a blow speaker but without hearing it, they think its a bad amplifier and they do not sell those parts anymore. However, they do sell a replacement speaker for $280. I don't know a ton about subwoofers so maybe you guys can help advise me on if its the amp or a blown speaker.

http://www.youtube.com/watch?v=r9dRpheHCdU