the amp sees the load which is on the other end. if it's 8 ohm, it's 8ohm.
12 & 18 dB/octave crossovers get all sorts of out of whack if you use the wrong impedance speaker.
6dB/octave crossovers just operate at a different frequency.
but.. if you have a 12db/octave 5000hz tweeter crossover designed for 4ohm, you can remove the low frequency bypass coil and you'll have a 8ohm 6 dB / octave 3500hz tweeter crossover.