If I remember correctly there is a 90 degree phase shift (between woofer and tweeter) for each 6 dB filter section. So, 6 dB is 90 degrees, 12 dB is 180 degrees, 18 dB is 270 degrees, and 24 dB is 360 degrees. Note that the 24 dB filter restores inphase signals for woofer and tweeter. Note also that with a 12 dB filter the same result can be obtained by hooking one driver up with wires reversed. There is no way to make a 6dB filter electrically "right", but perhaps the 90 degree phase shift is tolerable, and the benefits of a simple crossover offset the disadvantages. Also, the driver has its own natural roll off, perhaps 6 dB with a 90 degree phase shift, so that the net acoustical slope is 12 dB, with a 180 degree phase shift. Consistent with this, with 6 dB crossovers, the tweeter is often hooked up backwards.
The 24 dB design is not practical for passive crossovers because too many components are required. However, it is easily accomplished in a line level electronic crossover, and most of the electronic crossovers sold today are made this way.
The 24 dB design is not practical for passive crossovers because too many components are required. However, it is easily accomplished in a line level electronic crossover, and most of the electronic crossovers sold today are made this way.