Not completely sure I understand what you are measuring here, but the phase linearization FIR I included in post #538 is not part of the original FIR.
The original FIR is minimum phase, as is your IIR correction.
The phase linearization preset can be added to either the FIR or IIR correction and should give the same result in both cases.
Its aim is to linearize the phase of the acoustical crossover (ie DSP filters+passive components+box/driver/horn).