I own a pair of polks. Not the ones you mention though.
Just wanted to say they seem to be very reliable speakers. The ones I have have been in direct sunlight for like 10 years or more and still look and sound like brand new. The surround on these things is amazing.
I haven't installed the Crystal's in my wehicle yet (waiting for a few more things), but I half assed enclosed them and powered them up. They sounded great! Really good mid and bass response and the tweeter sounded excellent. It's titanium but didn't sound harsh even when aimed right at me. You can pick them up on Ebay, I don't remember the name of the seller. They were $193 to my door for two pair of them.
ETA- I heven't heard those polks, but thought I'd let you know the Crystals sound great and I've read a few other reviews and they speak very highly of the speaker. You can read one review here: http://www.infinitecaraudio.com/images/reviews/ssx615.pdf
People on here have a lot of good things to say about the Crystal SSCS 615 component sets. I think you can get them for around $150 or so. I would have gotten those, but I didn't want to cut holes in my truck since I don't know how long I'll have it. I went with the SSX series because of the good reviews and the tweeter is seperate from the woofer. Do a search for the sscs. From what I've read, they can't be beat at that price point.