The problem is that you bluetooth headphones likely have a microphone, which can only be set to 8khz. So Reaper will default all sound to that.
In MacOS you can select set your computer to use the headphones as output and internal mic as input, and then use "System default devices" in Reaper, or you can create a compound audio device and select that.
There are still potential problems though, my headset only does 44.1 kHz so any tracks in 48 kHz get resampled when I use it...
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