Contour Network Designer

This is the kind of "off the shelf" calculator that seems super convenient at first, but will rarely provide good results.
The reason is that it is calculating component values based only on electrical theory, not the real world response of driver + cab.
So in theory, XmH and YΩ may give a good shape, but this calculator has no way of knowing if that rolloff is in any way suitable for the actual project. Specifically, it doesn't know if the electrical impedance of the driver stays flat throughout the frequency range of interest, so the actual real life response could be quite different.
Albeit it's a lot more work, it's much better to get actual frequency and Impedance response files and model the results in something like XSim or VituixCad.
I find that site's 2 way filter designer very useful.

2-Way Crossover Calculator / Designer

Butterworth is the filter for flat impedance and power response:


So lets build it, time-aligned here IIRC:


Terrific impedance and phase alignment:




The 3:1 ratio of components comes from the hexagonal symmetry that hides in BW3:


Having got impedance flat, there is no reason you can't apply a contour to the whole thing at the amp output.

Since this is an 8 ohm filter, 4 ohms and, say, 1mH will create 3dB bafflestep. In practise you use 6 ohms for most drivers. So 3R plus 1mH.

Graphs from Boxsim: Software | Visaton