In a straightforward L-pad with just two resistors, the attenuation is frequency independent. However, Wayne's compensation network incorporates a capacitor in parallel with the series resistor, and this in turn adjusts the level of attenuation by frequency.Since you asked for mathematical models, everytime you reduce the voltage to a driver by half, you attenuate by 6dB. Level of attenuation is calulated by:
dB = 20*log(Vo/Vi)
where Vo/Vi is the percentage of the original voltage.
I think the easiest thing to do is to just use one of Wayne's crossover networks that he developed for his PI 18" speakers.
Gar.