If you upgrade the speakers like you have and basically add a mid and tweeter to the rear you are going to have a lot more mid and high range to your system. You still probably have the same amount of bass as you did before but you have a lot more output from the mids and highs because you have more speakers that play in that range now. This is why a lot of people will only upgrade the front speakers so that you have an improvement in the front tonality while still being able to keep something near the same balance between mids, highs and low end. In your case even adding an amplifier is not going to improve the situation more than slightly becasue while you increase the bass you are also increasing the mids and highs almost the same amount. You would be better suited to get a moderate amp and power a single 10" sub in the trunk. This should give you the amount of bass that you are looking for.
