Per default Capture tracks all channels in mono and the Capture Master track in stereo.

On playback, provided that all mixer inputs are set to digital input, Capture returns all tracks to the channels from whence they came, and the master track to whatever the hardware uses as the default stereo return (depends on the mixer).

So by panning the playback channel input, or perhaps by recalling the mixer settings from the recording session, you should be able to get a good stereo image from the mixer's main output.

You can also try to figure out where the Capture Master is returned to on the mixer, make sure that that's set up as a stereo input, and route it to main out on the mixer. Provided that it was the mixer's main out that was recorded on the master track it should reproduce as recorded.