I'm not sure if I followed the question 100%, but it seems like you were asking if you could run a sub without upgrading from speakers. The answer is yes, as long as your headunit has a subwoofer out channel, just put your RCA's to the sub output, hook up those RCA's to your amp, and wire your amp up and bam, you have submoofa.
Hope I understood the question correctly //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif