I have polk db651s in the doors, polk 5x7 coaxials behind my seat, alpine deck with alpine power pack and it sounds bad ass. a speaker with 40+ rms watts needs an amp for sure to sound better.

 

Going to a separate with the tweeter firing closer to ear level will help. Also speaker efficiency is the name of the game when using the HU amplifier. I am using Hertz DSK165 and they is plenty of clean sound at any speed.