two things:
1) what simulator are you using to predict the transfer function of your mods?
2) the simulation will be greatly improved if you provide it with the actual impedance data and FR data of your drivers

I have been doing much the same kind of work here with various Altec components, but I have the imped and FR data loaded into Jeff Babgy's PCD. Jeff told me that you cannot predict FR from just looking at the impedance of your system...he's a guru in this, so I believe him, but I am also still searching to gain a better understanding of why this is. Specifically, for example there are calculations one can perform to calculate attenuation by knowing the dB cut value and a tweeter's nominal impedance. So in my mind there is a definite mathematical link between imped and FR of a system.

Do you have a schematic of your XO that you're working with?