There are several things that can cause distortion or

"clipping"

1.

Amp not powerful enough for your sub ie.

100w amp powering 500w sub

= distortion when volume is turned up.

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.

This can cause damage to the amp and sub
2.

Sub to big

/ small for size of box
3.

Sub not matched to box i.e the sub requires a ported box and you have a sealed box
4.

Crap componets seems unlikely in your case though
5.

Bad connections from stereo to amp or amp to sub
6.

Too much gain on the amp

could be any of them really.

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need more details!!

what head unit are u running as well?