are all of these interfaces showing on the list of available audio outputs in Plexamp settings? (sounds like you’re switching between default devices at the system level, which should work … just curious if the midi-created interface is recognized by Plexamp)
after switching from one default device to another at the system level, does the issue persist if you restart the Plexamp app?
posting Plexamp logs (~/Library/Logs/Plexamp) might help the devs look into it further
