Jason and Adrian, it can also be pointed out that selecting an "all channel" mode(even all channel mono)doesn't in itself create additional power requirements. When more than one sound source is playing simultaneously, the sound power level is increased by 3dB for each doubling of the number of sources(1 to 2 speakers, 2 to 4, etc.). This relationship is illustrated here
The bottom line is that to keep the same overall sound level with, for example, 4 speakers rather than 2, each speaker would require only half, i.e., 3dB less, power, so no more total power is used with 4 rather than 2 speakers playing simultaneously. If the total sound level was to be increased, of course more power would be used, but this would also be the result if only one speaker was used to create the higher sound level.