You would deffo get an increase in volume by amping the rear 6x9's, but I wouldn't personally take that route as you'll shift the focus of the sound to rear of car, which isn't ideal, as you sit in the front!
I would look at running a sub, which will not only allow you drop to new lows(!) but in turn will allow you to filter the sub bass from the rest of your speakers, allowing you to drive them harder before they distort.
I've never had any experience with the amp/driver unit in one box, so I can't comment on sound. They are very convenient, no doubt about that, the only thing that bugs me is the lack of upgrade path; If you run a sub and amp and amp combo, you can upgrade in stages, replacing one, then the other, rather than having to find the cash to start from scratch again.
As someone who only has half his boot left, I can understand why you don't want a massive enclosure, but there some tiny, tiny ones out there, designed to house speciality woofers that will run in small enclosures - I have a buddy who's running a 10" JL in a very small enclosure and it sounds terrific.
After that, I'd look at ampingt the front components, and then look at changing them to something that will handle masses of power and give a good signal to noise ratio. Your 6x9's will be more than adequate to sit on the back shelf and chug away off of the head unit
