Dorokusai,
Being a Polk Audio CS rep, I'd like your opinion on the notion that certain Polk speakers really need external amplification to sound their best. I don't think this would be off topic as the OP's question does imply that he also thinks he needs an external amp.
I'm not one to buy and/or audition dozens of speakers and the new RTiA line is readily available locally and definitely meets my requirements for living room aesthetics and size (the center in particular), though I haven't actually attempted to listen to them yet.
From the specs they seem to be typical 8 ohm speakers but I've heard hearsay that they spend a lot of time in the 4 ohm or lower range and that is why some receivers may struggle with them at high volumes. Do you know if this is true? I'd be looking at the RTiA 7 although I may cheap out and go with the RTiA 5 because it has fewer woofers and I already have an SVS SB12-Plus sub.