there are many factors that could be possible... sensitivity difference... bad RCA cables... fader not even on the deck...

Try this, swap the RCA cables on the amp... use the front cables into the rear inputs, and the rear cables into the front inputs... see if there is any difference. (this will check the RCA cables) You can also try swapping the speaker cables... again, front to rear and rear to front... (this will check the amps channels)

Check the fader on the deck.

Make sure everything is wired properly... positive to positive, and negative to negative everywhere.

The Kenwoods may just not be as sensitive as the Polks... in that case, just turn the front gain up a little, or fade to the front more...

Also, the front speakers in that car sit very low, and point at your feet... which does not do anything as far as getting the sound up to your ears well... Components will help out here, as you can mount the tweeter higher in the door panel.

Theres a lot of variables... check some of the thing out that I mentioned... they are all possibilities... but I bet its just because the fronts are soo low in the door. I had the same problem in my car, untill I upgraded to front components.