I agree to the constant voltage in asmuch as it leads to a constant acoustic power which lead to a constant SPL. Electrical power will not be constant because of impedance variations of the speaker(s), however constant acoustic power is what we want inorder to get constant SPL. Of course, the desireablilty of high frequency beaming in order to reduce reflections to improve imaging and such will even modify the desirability of constant acoustic power.

Paul