I couldn't get the window crank off.
There is normally a little "C" shaped loop on there. They make fancy pullers for this but you can just be careful with a screw driver and pop it off.
All things being equal, 6.5's will normally give you more midbass then 5 1/4 speakers. But what other factors influence overall volume? Well we have a few major ones including brand/model, applied power and installation.
As far as brand/model...I have no idea. It really depends upon how much $ you want to spend, what type of music you listen to, how much work you want to put into your install, etc. Throw up these details and people will be able to give you some good ideas. Additionally, components sound better but they are more expensive.
Everybody here will agree you should be amping your speakers. Amplifier are much cleaner and more powerful then head units. It should be noted though that you only gain 3 dB by doubling your power, so the jump from 14 watts to 28 watts doesn't mean you will go from 125 db to 250db.
The most important aspect of your system is the installation. A little sound deadening, secure mounting, etc will equate to big gains in output and clarity. Do some reading about how to properly install stuff. Additionally, some people like having a good sound stage/proper imaging. I won't go into this...but this is determined by your install.
It was suggested you ditch the 6x9's in the back. I wouldn't do that at this time. Normally I make recommendations against rear fill, but but since you have already have them I wouldn't take them out. The reason for not running rear fill is (1) you have more money to spend on your front speakers and (2) it is normally bad for imaging.
Hope that helps. Throw up some details about your budget, music, etc. and we should be able to help you out with some drivers.