if youre going off headunit power, id say all 4 would make a good difference, if you go with an amp you can cut out the backs and compensate with a good set of fronts, but as it is now, id say changing them would do you some good if you want volume. . .
the Db is Polk's entry level so itll make good use of the power given to it without breaking the bank, them and the Infinity reference are my choices for low power and low budget, Focal Access 165A1 would be my main choice with a higher budget (the polks and Infinities sit at around the 200 mark, while the Focals stickhang around the 300 mark). . .
and as for the amp thing, STRICTLY speaking no matter what aftermarket speaker were talking about, itll never live up to its full potential without a good clean off-board amp supplying them power, but some are more efficient than others and will make better use of the low power of the headunit's internal amplifier
