I would say if your going to be using Polk for your left/right speakers, you want to get a Polk center. Like you said, you want the speakers to be voice matched, especially for the front stage. To answer your question though, there is nothing wrong with Axiom centers.
The Axiom Qs8's are quadpolar, meaning 4 drivers all firing in phase. This simulates multiple bookshelf speakers lined on walls of movie theaters. Not familiar with the polks, but if they are bookshelf speakers, bipolar, or dipolar, they are a different design and won't give you the same experience.
The Q's pretty much envelope or wrap you in the experience, a bookshelf can't do that.
