Not necessarily, dadd. Although Andrew didn't specify exactly why the rear drivers would use less power, it may be that part of the DSP processing is to set the back output at a slightly lower level than the front. If this was on the order of 2dB it would explain the 60% rule of thumb and would have nothing to do with the relative number of front/back drivers. In order to maintain the correct front/back balance the gain of the amplifier channels would have to be identical(or a level control to adjust the outputs would have to be available).

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Enjoy the music, not the equipment.