If the Polk 12s are the ones I heard with built in subs - they're plenty good. If they match your current speakers sonically, and are not too expensive, that seems the most sensible way to go.
If you want to switch brands, the Axioms are great - much more lively than the Polks, better for HT IMO, will definitely get you a sense of "I'm in the room with the performers", and they're available in almost any finish imaginable... but don't expect them to sonically match your current Polk surrounds.
G'luck.
