I have a 98 Tacoma extended cab with four speakers. I replaced the head unit this past summer and that helped out the sound quality quite a bit. Now I'm looking to replace both the front and rear speakers. While doing so, I'm also thinking about adding an amplifier to the mix. Will the amplifier provide a significant change in sound quality? If so, can I get some recommendations on which ones to purchase? I may add a sub later on, but as of right now I have no plans to. Also Crutchfield is in my hometown, so I'll pretty much buy everything from them.