Notch filter design calculator

Rather depends on the design context: except under the same conditions as the original measurement was taken or on a baffle (sometimes not then either), the LF impedance characterisitc will change as it's affected by enclosure load. Variations can be useful; Joe uses one with the Elsinore to flatten the upper vented box impedance peak -others, self included have done the same in various designs. I don't think many have done so for a free-air or equivalent however.

DSP doesn't change the electrical impedance of the driver / speaker system load, it 'merely' manipulates the input signal.
 
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that's a ("Series") impedance - notch circuit with 3 components in series with each other "across the driver" - useful with odd order passive highpass filters to limit interaction effects of capacitor reactance vs. a driver or system's Z-peak.

the ("Parallel") case with R-C-L elements in parallel with each other and the composite filter in series with the driver is used tame frequency response peaks by presenting a high impedance at a specific center of frequency.

Measurements are your friend.