If I were to just get a Sub thats 300-500 W RMS, would I need to just get an amp for this or do i need to change my whole entire system.... all i really need is bass , I find the stock system if fine but lacks on base.. any info suggestion is great.... iam low on cash at the moment so spending hundreds or thousands on a complete system such as 4 speakers, deck, amp , sub is out of the question, just a sub that I can run on stock deck and maybe an amp if need be.