Rep Power: 0

I keep my bass at -6 so it won't sound distorted. But as soon as the bass hits at anything higher than -6 the sound is really distorted. What causes this? Is it the amp, speakers, sub, anything else?

 
Rep Power: 195

Depending on the music and volume level, if you can't have the bass at 0 (flat) then your speakers are most likely damaged. Any stock stereo should be able to go near full volume with the bass level flat.

 
Rep Power: 518

I have not really seen speaker issues here, but I've seen a few with rear shelf rattles that make buzz and fart noises.

Even found one with a piece of broken plastic (came off the shelf trim) bouncing in a speaker cone.

One of the seat belt reels in my car makes a buzz noise when the radio is on low.

 
Rep Power: 187

Is it an Si with the stock woofer?
I've heard them and IMO they sound like crap.

The first time I heard one I thought it was busted.

(But I'm spoiled by mid to high end audio equipment)

Could get a multi-meter and check the resistance of the speakers, I believe all the speakers in the entire car should be 4 ohms.

anything less than 3.8-3.9 ohms on the woofer would indicate the coil is certainly damaged.
(Should really be at 4.0, but i assume there is some play, especially with the stock woofer)