OK, according to page 108, the centre channel is simply the sum of the Front Left and Front Right channels divided by 2 after the fader coefficients have been applied (i.e. fading to the back will decrease the volume of the centre channel.

The mono subwoofer channel is calculated similarly and is (FL + FR + RL + RR)/4 but taken from before the fader coefficients so they don't affect the subwoofer level. After that a 4th order low pass crossover is applied.

If using left and right subwoofer modes (no centre channel available) the left subwoofer channel is (FL + RL)/2 and the right channel (FR + RR)/2. The low pass filter is only 2 order though for each channel if you use stereo subwoofers.

So they are simply averages according to the doco which I dont see being much problem on the subwoofer channel but I can see being an issue with the centre channel - constructive and destructive interference will be happening I think.