I have the option of either placing one speaker on my rear wall (6.1 surround) or two speakers (7.1 surround). My speakers are cinema i (center), ascent i (front), clarity (sides), fresco (rear). Question is: since there is same signal going to both rear channels, will i notice a difference if i have 2 rather than 1 speaker back there? Will there eventually be "true" 7.1 dvd's? Also, what is recommended height for the rear speakers?