Here is how I changed the output from channels 1 and 2 to the stereo channel 15/16 on my Mac,
I first used Presonus Universal Control and activated "D. In" for channel 15/16. Then I used Audio Midi setup on Mac to redirect the standard Left/Right to channels 15 and 16.
In order to do this: (Finder > Applications > Utilities > Audio Midi Setup), select StudioLive 16.0.2 audio interface
Select "output" tab then "Configure Speakers ..." button bottom right.
Voilà, you can select which channel to use for your left and right speakers.
I apologize for Windows users, I don't know how to do it.
Hope this helps,