From a discussion on Linux Musicians Forum to see if there is a linux equivalent of Voxengo CurveEQ, crocoduck suggested Fons Adriaensen's JAPA.
A comparison of the frequency response of an Oktava MC-012 Omni and Oktava 319
The Oktava 012 is Input A.
The red line in the graph is the difference between the inputs (A/B).
The blue line is the frequency response of input B.
More info on JAPA here