Hi,
Since RF-82 is 8" double driver i would advice to set it to large . The centre is always crucial but in your case you opted for a small sized centre with respect to the front's. To avoid exessive cone movement set the centre to small and the rear's RS-52 as well. This will lessen cone distortion and leave the 12" sub to handle all the lows in phase with the front's .
Beacuse the front's are set to large, wideband signal is transfered to the fronts. So maintain 0 degree Phase on the sub to avoid roll-off's. I am asuming you have taken care of the room acoustics well. Set crossover frequency to 80 Hz .
Hope this will do.
Regards
Rajesh