Answer
Many vocal microphones have a rise in their high-frequency response. For example, an SM58 has a frequency response that rises up to 5 dB between 2 kHz and 10 kHz. If these microphones are used the resulting room tuning will have a corresponding dip. A microphone with fairly flat frequency response will provide good results.
There are a number of measurement microphones available at reasonable prices ($100 or less) and it isn't necessary to use a precision, calibrated microphone.
Note
The Talkback input of the mixer has phantom power to support condenser measurement microphones.