We had some discussion about this topic recently (after the mains, get a sub or a center or surrounds next?).
I think bigjohn is right; especially if you have the Polks for surrounds already, I would get a decent sub before either the center or new surrounds. You can always "phantom" the center channel.
More on-topic, however, surround processing has changed dramatically over the last 15 years. In the old Dolby Pro Logic system, the surround channels were neither discrete nor full range. Nowadays, (Dolby Digital Surround, et al), the surround channels are mixed very specifically. It can be a very rich experience!
Can you get by and enjoy a new setup with the Polks as surrounds? Absolutely! AND I really adore the QS8's. They sound wonderful, and provide a very enveloping movie experience that, to me, in my listening environment, cannot be matched by direct-radiating surround speakers.
