Unlike common Linkwitz-Riley filters, many other high & low-pass IIR &/or FIR crossover filters don’t sum to unity or have matched phase.
For a perfectly flat summed response, FIR Designer can calculate a COMPLEMENTARY HP (or LP) filter from the chosen LP (or HP) filter, when set to linear-phase.
The filter pair can be used for targets in multi-way designs, providing more crossover options. Works for Bessel, Butterworth, Chebyshev I & II & Elliptic filter prototypes. (FIR Designer also has a variety of linear-phase brick-wall crossover filters.)
