[SilverSleeper]

As far as 2v vs. 4v output voltage:
well, with the amp at the same input level settings, yes, the 4v will be louder. But, the main reason to step from 2v to 4v is to reduce the amount of amp gain needed to achieve a loudness level.
Increasing output voltage while decreasing amp input gain will get a stronger signal through your amp and a cleaner sound.
If you dont understand what Im saying, just go turn on your home stereo amp without any signal (ie, no CD or radio playing). Notice that the more you turn it up, the more static you hear. This is the equivalent of turning up your car's amp gain. You turn up the input voltage so that you turn your amps gain down and reduce that extra noise.
You need to match your pre-out voltage to your amps input level. If you overdrive your amps with too much input voltage, it will distort your sound. If you are using your speaker outs without a converter to drive your amps, thats a big no no.

As far as your amp and speaker setup:
Why would you deck power your front speakers, and then amp the rears and subs? You will get a lot of your mid-high frequency information from your rear speakers, and the fronts will not even be heard. I guess if youre used to that, then thats your choice. That could be a reason you say it sounds too quiet. Its gonna sound weird. Generally speaking, your fronts provide your mid-high information, and if you have rear speakers aside from subs, they are there for fill only, and are not as driven as the fronts.
Now, if you are using your 6x9s for mid bass, thats different. You would want to cross over those to keep your mid-high info out of them and up front to your Pioneers.

You have a good start though. good luck!