Donny
You can go into menu/voice preset/choose vocal harmony from dropdown menu type then touch yellow selected harmony type.
from there you can set the seperate volumes for each voice then hit upper right dropdown menu to write and name preset.Exit and then us assinable knob to ajust harmony volume. I don't know if you can disarm assinable knob but you can make another preset and name it something else llke DONNY2 and you can then choose what preset you want without touching the knob once you have that set to the correct level.
Mel