Ok so Iooked up your cars sound system and it does have an amplifier, 170watts. I also looked up reviews on the Polk components you bought and almost all the reviews say the tweeters are very bright, and the crossover needs to be set to -3, which you did. Regardless though, the tweeters are gonna be bright.
The factory head units sound processor likely was tuned to the speakers it came with, I'd recommend a new head unit if anything. The ideal would be a head unit and amp, but you said you weren't keen on that.
Edited by spedanden, 06 August 2012 - 11:22.