Ok...
This is what I would do. If you already have rear speakers, keep them. Run them off the head unit, its rear fill dont worry about it.
Front speakers, get a resonable set of splits, your ears will thank you for it $300 or so. Get some sound deadener, and wack lots of it in the front doors. This will help your midbass. See the front are meant to be able to play mid bass too, not just mid. So this will help mearg in the sub bass better. As technically all sound should be coming from in front of you... Like a stage!
See, a sub is just for that, sub bass. Bass that is non directional, say 80hz and below. And its only as bassy as you turn it up, I honestly cant see you needing that much power, I got a 300w 10inch JL and I've never really streched its legs and its easy loud enough.