Personally i would suggest getting an amp, as i replaced my front componants and put 6x9s in with new head unit and it ended up quieter than the original stereo, so i went straight to the auido shop and bought a nice expensive amp, and very glad i did.
Since then i have added a sub with independant amp. Goes so loud now if you dont crack a window/sunroof the pressure builds up in the car and pushes on your ear drums as if you are under water. Made me go dizzy once, not a nice feeling, so i dont have it as loud as that now unless i have a window open.
Absolutely love my stereo though!