Speaker model will probably be Focal Chorus 726's, Focal Aria 948's, 936's, 926's. Or the Wharfedale Diamond 250's. I am not decided, waiting for a special to come.
Below is the worst case scenario:
Aria 948 sensitivity..................................... 92.5 dB/2.83V/1m (3.28 feet)
Impedance.................................................. 2.5 ohm (that's the minimum, likely occur in a narrow band between 40-400 Hz)
Sitting distance (13-14 ft so say 13 ft)...... 14 ft
Room gain................................................... Assume none
No. of speaker............................................. Assume 1
Room dimension, 12X24 ft, assume height is 8-12 ft so that's considered a large room by THX standard.
Based on above information, you will need approx. 1,036 WPC. Note that Focal recommended 50-350 W
It would be reasonable to assume you will have room gain of at least 3 dB, and another 3 dB gain for having two speakers making sound about equally loud on average most of the time, then you will need:
1036/4 = 259 WPC
Now, let's assume the 2.5 ohm minimum is just a sharp dip at below 100 Hz that the subwoofer will take the blunt, then you can based on the requirement on 4 ohm nominal, that will bring the power requirement further down to:
= 259*2/3.2 = 162 WPC
That's the average power, or the so called (incorrectly) "RMS" power, so in reality, 81 WPC will give you the 162 WPC peak.
Still, conservatively speaking, if you insist on getting those multiple drivers with low impedance dips, need high spl, and sit 14 feet from them, you should get an external amp rated at least 200 WPC 8 ohm, 300 W 4 ohm.