Just to confirm, are you looking at the Mixer layout of the Multigrid? And are you trying to toggle the widget named "ToSubmix" (or whatever you named your submix)? The one that is a potentiometer by default? And you've changed that widget to a Toggle-Fill?
You could try something like this: Add a SoundToGlobalController to a new Track and in that Value field you could say:
!cc88 asToggle (or whatever your Akai is sending)
and the GeneratedEvent would be
Then use the gear menu add !ToSubmix to the shared event values.