My take on Polk
I'm driving my CSi5, RTi10's, RTi4's, and FXi3's with a comparable Denon 3805 and it's about all my receiver can dish out. I'm in the market for a separate amp, so if you ask me, I'd recommend some 8 ohm >92dB spl speakers for your Yamaha 2500. The Polks are awesome speakers, but you'll never realize their potential without a beefy separate amp (or a flagship receiver).
Have you considered the Venturi DV62 CLRS center or DV62 SI bookshelves? The center channel is very comparable to my CSi5, with much better output due to it's efficient specs, plus I think it puts out better low end. The bookshelves are outstanding, and some guys even have the crossovers moded for $20 to make them compare to speakers costing several times as much. Lots of guys here have them, and I've yet to see one negative review. You'll flip when you see pricing.