Answers: Have you tweaked the cross over settings on the head component, amp and sub? - i.e. high and mid frequency are deliver to your normal speakers and the low frequency go straight to the sub?
Also you need two right amps (more amps for more subs) - one to drive the sub, one to drive the rest of the speakers. (make sure the speakers can take the power - you dont want the amps to blow the speakers).
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I would take it to an audio shop and they should know how to help. They want your business but I would be fixed to add on's not replacing. Limited to a bigger amp. and better speakers.
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