I assumed that you were originally asking about monitoring the Digitone via the stereo EXT IN connections. The EXT IN signal connections go through the AR’s compressor along with its internal sounds, so you can monitor the Digitone in stereo. See the diagram on page 13 of the manual. There are no settings to adjust.
If you want to sample the Digitone into the AR, connect the Digitone to the AR’s AUDIO IN inputs. I don’t know whether you will hear a stereo signal when you monitor the AUDIO IN inputs but any sample that you take will be reduced to mono.