If the impedance of a driver is measured, should you use the measured impedance at the frequency which you intend to crossover at for the calculations for the cap/ind. size?



Also, let's say hypothetically that a speaker's passband is very near its resonance. If the actual measured impedance of the driver is an average of 20 ohms across the driver's passband, will the amp always "see" 20 ohms and put out less power accordingly?

 

Don't overthink this one too much. Impedance in audio drivers is a hot mess and you will end up wronger than you might guess, unless you are measuring your drivers directly to get your data . . . and remember the T/S parameters will change over time, including resonant frequencies (and so impedance@frequency). Make a small effort to keep the impedance peaks away from the crossover frequencies, and use the nominal impedances of the drivers.