Theres's two schools of thought on multichannel systems. Most people use different amps and speakers for the "mains", center and rear channels as well as different amps. That's an easier way of getting there. The second school, to which I adhere, is that the best way of achieving a real 3D involving and integrated sound is to use identical amps and speakers for each channel (except the SW of course). Maybe it is not necessary but certainly it won't hurt.

Given your room's size I definitely recommend the pro fours.