I’m getting my live set together, and am using Ableton Live 9 on a Mac laptop, and want to use my Roland MX1 Mixer as the main output device. I set everything up as instructed in the (rather pathetic) manuals, but got no sound from the Mac on the MX1. It took me at least half an hour of searching manuals and reading posts to work out what was going on. After installing the drivers, setting the sampling frequency to be the same, putting the MX1 in mixer mode, connecting the laptop to the MX1 PC input, setting the sync settings still no joy. The MX1 will only play the main outs from Ableton if you set Live to output its master signal on channels 17&18, rather than the more common sense, preset, 1&2. So you have to set the master channels to output on channels 17&18. It’s not so easy to find out how to do that in the Live manual, but it is quite simple once you know, and solved my problem.
In the bottom right corner of the Ableton screen, there is a button labelled I-O which is probably greyed out as you can see above left, unlit. If you click on it, the button lights and the input output settings will appear. Set the Master Out to 17/18 and you should hear the Ableton master output through the MX1. I’m using this system in a couple of weeks in a Professor Chill set at the Samsara Festival 2018 in Hungary.