Here are some steps you can follow to hopefully have a rock solid stereo image. On the Dirac Live "Volume Calibration" page:
Keep the mic level at 100% with zero gain increase.
With all speaker levels at max, play all speakers and increase the master output gain so all speakers perform at comfortable level. Somewhere around 20 dB should be fine.
Find the quietest speaker and match this speaker to the other louder speakers. This is done by reducing their volume levels individually. Try to keep the values between +/-0.2 dB to the quietest speaker level value.
Increase the master output gain a little more to make all speaker’s levels a bit louder.
Run the measurements, design the filter how you’d want it, and then export the filter to see what it sounds like. The stereo image should now be "rock solid".