I’m playing back a piece that is setup with piano and singers, and I typically use NotePerformer as my playback engine. I will go to “play” mode, open the playback settings, override “Tenor (Ah)” with my vocal playback instrument of choice (an Oboe, for example), and it will play back properly… until I stop and start again. Then my change is reverted.
Am I doing something wrong? How can I permanently override the performed sound of an instrument without it reverting? I have tried a saved preset without luck.
Change the instrument of the player in Setup mode, the re-do Playback template > Noteperformer in Play mode? Note that you can edit the staff label in Setup mode, opening the player’s card and invoking the context menu on the instrument he’s holding… Nothing prevents you from calling a marimba a flageolet 
Which version of Noteperformer are you using? I had the same problem, and the latest version solved it - in the earlier version things would revert back to the original instrument, and voices would only sound as a group, not as a solo. All better with the latest iteration.