So I have noticed during testing that, when I add a second order time delay network between the tweeter and the cross-over, the response above 10 kHz changes.  That doesn’t make any sense at all – the time delay network should only change the phase.

The blue line below is the starting point of this experiment, and the yellow line represents the “solution” to this dilemma.

The solution?  Adding a Zobel to the tweeter to bring down the inductive rise in impedance.  How much of an impedance rise?  Almost nothing!

Maybe there is something else going on… I measured this multiple times and obtained the same result.  Measurement conditions?  The mic is 6″ away, output level set to -31 which isn’t too loud, no indication of overload on the mic pre-amp.