Testing Microphone input Linearity with REW

[Sliderule]

I would like to use REW to measure the linearity of the micrphone input of my Motu M2 USB audio interface. I did a bench setup using a HP 34401A multimeter to measure frequency and amplitude. Tek 465B scope and HP 3312A function generator. I am using an xlr to bnc adaptor to input. I initially set the input level to 5mv RMS at 1KHz. I can see the input on the screen and adjust it up and now. I have hoped to use the REW SPL meter to measure the output of the ADC. Unfortunantly it only reads the noise level of the circuit at 12dB. The product specs +0.0dB and -0.1dB 20-20KHz. Any tips on doing this if it is possible would be greatly appreciaeted.