As with any speaker that uses a passive crossover, some of your power is lost in the crossover itself. On top of that, speakers that use contouring networks, notch filters, impedance compensation, etc... all end up sucking up more power. While the end result might be a speaker that measures very well in terms of amplitude linearity ( flatness of frequency response ), the sound produced is typically lifeless and lacking in "dynamic jump factor". That is, unless one feeds them with GOBS of power.
As far as impedances varying with frequency, all speakers do this to some extent. SS amps without output transformers deal with this by varying their available power in large fashion. That is, big impedance swings result in big differences in available power. Tube amps and / or SS amps with output transformers are also susceptable to impedance swings, but the transformer itself acts as part of the load. This tends to stabilize the available power to some extent, but not enough to provide equal power at any given impedance. The output variance that does occur with impedance changes are what making playing with the various output taps on the amp worthwhile. Not only can this change the available power output, but also the tonal balance and distortion characteristics.
With that in mind, Usher's are designed by Joe D'Appolito. Joe is a speaker guru and i'm sure that the crossover is relatively complex and parts heavy. This tells me that the impedance may not swing all that wildly compared to some speakers, but that it may take considerably more power than the speakers that are simpler in design. If it were my system, i would not hesitate to play with the output taps and see what worked best for your specific speakers and listening tastes. Sean
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