1. Rz=4.2 ohms and Cz=20mfd

The cross-over frequency is 1925Hz (first order) as can be seen by the filter phase. The filter phase perfectly bisect the low-pass and high-pass roll-off to give precise imaging and transparency. Also the impedance phase is showing resistive loading after the cross-over frequency.

2. Rz=4 ohms and Cz=22mfd

Altering the zobel resistance and capacitance value gave the below response,

As you can notice even by slight modification of the zobel resistance and capacitance gave me a huge shift in the filter phase which is now around 1816Hz and does not bisect the low-pass/high-pass FR roll-off. Also the impedance phase gets slightly inductive before the cross-over value which could result in loss of imaging and transparency.

3. Rz=4.5 ohms and Cz=15mfd

In this case too there is a change in the filter phase now to 2234 Hz and it does not bisect the low-pass/ highpass FR roll-off. Again this will cause imaging and transparency issues and the low-frequency will be very loose.

Conclusion: Its quite important that the values for the zobel resistance and capacitance are calculated accurately and they are also in-sync with the high-pass filter response for getting the correct imaging and transparency. Even modification of the values by few fractions can play havoc with the overall sound stage.