Cat:
My GUESS (I don't have any knowledge of this one way or another) is that a replacement mic that's used in these types of calibrations needs to be the same make/model as the original.
Mics, like speakers, have their own FR deviations from perfection, and my ASSUMPTION is that when a receiver / processor manufacturer designs a calibration method, they're taking into account a mic's deviations from perfect flat response.
Replacing the manufacturer's supplied mic with a different one, even a much more expensive, "flatter response" mic, might just cause problems with the final calibration.
I would re-wire a plug (and maybe a new length of cable if needed) onto that one or buy an exact duplicate from the manufacturer.
