I just did part 1 of my "big" audio upgrade recently. Part 1 was high end speakers, a DSP amp, new wiring throughout and interior sound deadening. Part 2 will be larger/better sound quality sub and full door treatment.
Here's my take:
1. You need a
dedicated sub. Don't do under-seat.
2. Those speakers are going to be your next weakest link. Instead of amping them,
upgrade speakers and amp the new speakers.
3.
Sound deadening goes a long ways.
4.
Focus on the front stage.
Like Scott said, you need a sub. Nothing comes close to replicating what a sub can do. Without one, you're missing a large portion of your music. Give up the cargo space and get a proper sub. If cargo space is really important you could pay a shop ~$500 or so to build a nice box in the factory location and modify the interior trim.
Those Kenwood speakers will be better than OEM for sound quality, but probably by a very small margin. You could get MUCH better quality out of a $100 pair. Go demo some stuff at an audio shop if you can and focus on clarity. I found there to be a huge difference in quality once you break into the $100-150 price range and then again once you get to $300-500. I extended my budget quite a bit after hearing the difference and I couldn't be happier.
Sound deadening the doors will give you a bunch more mid-bass, but still no sub-bass. It's no replacement for the sub, but it should make the low end tighter and more accurate. It's a very good value investment since ~$20-50 in material and an afternoon can make a big difference.
I'm a huge proponent of running front speakers ONLY. My rears are disconnected entirely and I've got a pair of nice fronts run off an amp. When I go to a concert, I face the band and the sound is in front of me. I think it should be the same in your car.
If you have extra money and tuning time, sure, add in the rears. If budget or time is a concern, forget the rear altogether and buy nicer equipment for the front.
My current equipment:
Head Unit: Pioneer AVH3700 - was the best of the "lower end" stuff a few years ago
Front Speakers: Morel Virtus 602
Front Amp: Audio Control D-4.800 running the Morel speakers "active" meaning I control the timing and EQ of the tweeters separate from the mid-drivers
Sub: Alpine 8" Type R (soon to be two 10" Focals)
Sub Amp: Alpine MRV500 (soon to be JL500/1 monoblock)
Sound Deadening: Floor: Dynamat/CCF/MLV; Quarters/ceiling: Dynamat/Hydrophobic Melamine Foam; Doors: nothing yet - ran out of time
My recent speaker upgrade has been phenomenal, but the sub addition years ago was the real turning point. There's just no "sub"stitute for a subwoofer.
Good luck!
