Jeff,
When I did my system, I was in the same boat as you were. I removed all the stock kenwood speakers and replaced them with the DB-651in the cabin. I put four MB Quart in the tower. I also put a 10" boaton acoustic G3 sub under the driver dash. I purchased the sub enclosure from www.audioformz.com All the cabin and sub woofer is being power by a Kicker MX750.5 the fifth channel is pushing the sub. The tower speaker is being pushed by a Kick MX350.4. I also changed the HU. I got rid of the stock one and replaced it with a unit that has 3 RCA outputs. One for the cabin, one for the tower, and one for the sub. I am very happy with my system. And it sounds awesome at 80' back. Look at my pictures of my stereo install. I have had it in for 2 seasons and ZERO problems.
Tazz