The MONO/STEREO switch, on the rear of the AD-10, adjusts the output of the XLR DIRECT outputs.
This switch changes the XLR Outputs from STEREO to MONO (DUAL MONO).
The differences in sound will be most noticeable and audible when using a panning sound or panning effects.
For MONO (DUAL MONO): The same mono audio signal will output from both left and right speakers, thus dual mono.
For STEREO: A typical stereo output setup would be to route both XLR direct outputs, to a mixer on two separate channels, and have those two channels hard panned left and right, for true stereo output monitoring.