I've never auditioned the RTiA9 speakers, but Polk's Web site claims they have an efficiency rating of 90dB. That's relatively high, meaning they don't need as powerful a receiver as most speakers, which often are in the mid 80s. Unless you have a large, highly sound absorbent room, just about any modern receiver should be able to drive them to decent sound levels.
The traditional rule of thumb is to spend about 2/3 of your audio budget on speakers and 1/3 on electronics to drive them, suggesting that it would be reasonable to keep your expenditure on electronics under $1K. A pre/pro plus external amps would be closer to $3K. If you want to use an external stereo amp to drive the front speakers, you need to get a receiver which includes pre-amp outputs. That'll push the cost of the receiver to near $1K.
For the best sound, you need to consider receiver models which include some form of room equalization, preferably Audyssey (Onkyo and Denon). The competing receivers with EQ and costing $1K or less (Pioneer and Yamaha) don't equalize the subwoofer channel.
Note that my cost estimates are based on list prices. Significantly lower prices are often available if you're willing to use refurbished equipment.
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped