We recommend the MiniDSP Umik 1, which is a usb microphone so no need for a preamplifier. This mic gets two individual calibration files, one at 0° and one at 90°. But other calibrated mic can do the job. Even better if you get a random incidence response for your microphone.
Preferably use the mic vertically and upload the 90° cal file. Ideally the right calibration would be for random incidence but more often you only get 0° and 90° responses. With a Umik the random incidence is nearly 3dB higher at 20kHz than the 90° response. It means that if you use the 90° calibration, you are a bit over-correcting and the real response of your loudspeakers is about 1dB lower at 10kHz and 3dB lower at 20kHz. See hereunder corresponding graph from IEC standards. You can see some examples on measurement mics websites such as GRAS or Bruel&Kjaer.
One type of measurement may be spoiled by cheap microphones : because some microphones have have quite a high intrinsic noise, the distortions figures may be hidden into the mic noise floor. This can be improved if measuring at a higher sound level (but be careful with your ears and louspeakers).
