I think you should try powering them off the head unit first and then move to an amp later. I think you'll be surprised. A 6x9 has the same surface area as an 8" sub. You need a 6x9 sub over a mid range and tweeter 6x9 because they won't play as low. You can use a thin bead of duct seal all the way around the speaker baffle so it seals against the rear deck.

Also, since these are designed for "infinite baffle" they will not take a lot of power. Which isn't bad at all...