What would cause that in the first place? Are you thinking about the BAVSOUND subs?
I think a previous owner has really liked bass. Going by whats on the net its quite common. Rockford Fosgate do direct plug and play upgrades, so Ill go with them if they can't be repaired.
Not really a massive car Hi-Fi connoisseur, but the Individual audio system does sound good, mine though, currently sounds like its coming from my I-phone speaker.