I am thinking of buying two new Polk Audio DB651 speakers for my car, a 2004 Honda Accord sedan. I don't really want to replace the factory speakers in the car because they are decent. I am probably going to get separate enclosures for each of the speakers and put them on the floor in the back row of seats. Should I get an amp, if any, for them? If so, what would be the best amp for these speakers? (preferably the amp would be less than $100)