+1 on the Alpine Power Pack. I have an Alpine BT Head Unit and got the plug & play Alpine Power Pak. I am really happy with it. PO had older Polk DB series speakers (Component in the soundbar, 2-way in the dash) Still not happy with the sound quality AT ALL (though with the Power Pack the sound is very clean and can be cranked WAY up) so I am having Crutchfield's install and tune x4 - Polk MM series 2 ways and adding a Kicker HS10 self-enclosed, 10", micro-sub at the beginning of February.
Just a quick tip - VERY IMPORTANT to get enclosures (stuffed with Poly-Fil) for the 5.5" rounds, especially if you do not have a sub.