It looks like it's now possible to disable this macOS-CoreAudio-specific double-buffering ... see here:
https://support.apple.com/kb/PH13675...ewlocale=en_US
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I/O Safety Buffer checkbox: When selected, MainStage uses an additional buffer to process audio output streams, providing a safeguard against crackling noises that may occur when using very low I/O Buffer Size settings. If turning on this preference does not improve things on your system, disable the checkbox and select a larger I/O buffer size setting.
Note: Use of the I/O Safety Buffer preference increases the output latency and therefore the round trip (input plus output) latency.
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Justin, are you going to add this tiny little, but very powerful option, in Reaper 5.50 ?