If so, does this mean I need to use an adaptor to play the output on stereo headphones? My Stereo-Mono switch does not produce a two-ear audio. Would this be solved with a converter? Would be nice to receive other pilots and ATC in both ears.
Yes. The idea of listening to anything more than scratchy AM radio via your ADF was an alien concept in the 70s, unless you had some custom system made up for you. It's really only taken off since ANR headsets have made it quiet enough within the earcups to make music worth listening to.
You just need to get a mono-to-stereo jack adapter to hear the mono signal in both ears.