I finally got some time to try doing IEM measurements on QA490 (previously I was only using it for impedance measurements of headphones). It appears to me that gains of left and right mic channels are differ by about 4 dB. To verify that I used the same IEM, coupled it with the first mic, saved FR as an overlay, re-plugged cable going to QA401 L+ input to Out R of QA490, coupled the same IEM to the right mic and adjusted it to make sure that the traces look similar. As you can see, they indeed look similar but shifted by about 4 dB:
This looks like a calibration problem with QA490 to me. Is there an easy way to fix it, without sending the unit back for calibration?