Have you been out and listened to speakers other than the Polk's? Lots to choose from and they all have their own sound,...some do sound similar to others. If you can give us a better idea of the sound you like we could give you some better recommendations.
Off the top of my head, for your situation, I'd say skip the sub and get a good pair of speakers. And get the rest later if you feel the need and/or have the funds. If you add the price of the sub to your speaker budget you'd have ~$1000 for mains.
I'd also suggest you pick from a line that also has a center and surrounds available to be purchased later. So I'll give you three to get started with (and I'm sure you'll get lots of suggestions 'cause there are a lot of good speakers out there.
RBH Sound MC-6CT or
TK-5CT 
Aperion Intimus 5T
many, many others though, so,... get out and listen to some
