So I finally got back to looking into this today... I think that part of the problem is that I changed too much all at once. I basically changed by head-unit, my front speakers, removed my amp and added an under-the-seat sub all at the about same time - so I've changed a LOT at once. :-)
I put my stock tweeters back in today and it does seem better, but since my current head-unit (the Dynavin N7) has a 9-band EQ, I have to get the EQ dialed in to my liking and spend some time listening to it (whereas originally, I had a Rosen GM1010, which just has bass, mid, treble and BBE - as well as the Alpine 45Wx4 amp). I have a feeling that I'm just not going to be happy with the current component setup.
As I was looking around online, I came across this JL Audio "tip":
Basically, it recommends that you either use a component set with the woofer and tweeter right next to each other or a high-quality coaxial speaker in the door and then use an "imaging" tweeter - this way the woofer/tweeter sound isn't so "separate", but you still have the benefits of a tweeter in the a-pillar for "imaging". Which is basically what I started with. :-)
I think that I just don't like the sound from the door woofer and the a-pillar tweeter being so far apart from each other - it just doesn't seem to "blend" well. Maybe they're just too far apart in the 8th gen Impala for components to be beneficial with the stock mounting locations. Honestly, if you're going to keep the woofer and tweeter right next to each other anyway, I'm not so sure why components are supposed to be "better" than *good* coaxials - the Pioneer D-Series coaxials (TS-D1702R) that I have basically have the same crossover as the TS-D1730C components - it's just that they are mounted directly on the speaker instead of being in a separate box...
If you think about it, the stock speaker setup was a "full range" speaker in the door and a tweeter in the a-pillar. The stock door speaker is sent the full frequency range, which helps it blend with the tweeter better. Or maybe I can just get rid of the crossover and feed the Pioneer woofer the full frequency range - maybe that will allow it to "blend" with the tweeter better?
Or maybe I'll just go back to where I started - the Pioneer coaxials and the stock tweeters. :-) Combined with the 9-band EQ in the Dynavin N7 and the under-the-seat subwoofer, I think I can probably get a better sounding system than I can with the Pioneer components.
I'll probably eventually swap out my Alpine 45Wx4 amp with that new Kicker amp soon - that Kicker amp just sounds so awesome...