I'd would say that the shipping costs would be well worth your while. I think you would be pleasantly surprised. It's been a while since I heard the RTi6 from Polk but I'm guessing you will find the Axiom M3's will do a much better job of satisfying your new quest for clarity while retaining a level or warmth that you might be used to with the Polks. Polk, doesn't seem to want to post a frequency response graph for these speakers so it's impossible for me to say much more scientifically unless I heard them side by side.
In general though I'd take Axioms concentration on research over Polks concentration on marketing, anyday.
In the end, buy what you like, you can prove a better speaker on paper but which one you like better is totally your decision.
Edited for a PS:
I forgot to ask if you use a sub? The Axiom M22s, although slightly bigger, would bring you up a whole new notch in your quest for clarity. Pretty much as high as you can go in a bookshelf. However, they concentrate so much on detail that you need a sub to balance out the deeper bass. If you are not planning on using a good sub, stick with trying the M3s.