I have an 8 ohm L-Pad feeding a 16 ohm tweeter. The high-pass crossover is before the L-Pad. What is the effective impedance seen by the crossover? Is it 8 ohms (the L-Pad) or is it 16 ohms (the speaker) or something in between?
An L-pad is not neccesarily mantains the drivers nominal impedance. Depends on what the crossover's developer wanted to achieve.
The best way to tell what's happening is to measure the driver's impedance response and simulate the crossover + L-pad with an adequate software.
Whenever I hear these discussions about the fine points and micro-adjustments of passive crossovers, I wonder why anybody would fuss over this concept that is just a crude tool in real-world music listening? If accurate results is your goal, you don't want a passive crossover. If economy or simplicity is your main goal, then OK.