The 255c-RT for center and 4 80F/X-RTs for the surrounds. I don't have any listening experience with the Definitives. I have had some issues with the Polks but that is related to my particular setup (the speakers are in a custom cabinet, not a wall as designed). I have another theater zone with the 65-RT and they sound great. If you are not an audiophile I'm not sure you'd notice the difference between the Definitives and the Polks. There is also the Polk 265-LS which is a step up from the RT line. As with all speakers, it will depend on how much power you are sending to them. Under powering any of them will lead to underwhelming results. The quality will also depend a great deal on the room you are putting them in.

For the sub, I'd recommend two subs for a flatter response. I recently added a second sub and it improved the sound quite a bit. I'd go with two 10/10s over one Reference but the price difference isn't all that much. I believe you can power two subs with one SubAmp 600, though. You could also prewire for a second and add it later if you aren't happy with the results from one. The SubAmp is required with the in-wall subs as they are unpowered. Home theater receivers generally do not have an amplified channel for subwoofers - only an RCA preout. Most external subwoofers include a powered amplifier.