When I instantiate Scaler 2.73 within Blue Cat Audio Patchwork (to enable the use of multiple instances of Scaler within Studio One 6), I am not able to pass MIDI to other VST/AUs within the same Patchwork instance. However, when I instantiate the VST3 equivalent, it passes MIDI successfully.
The AU spec does not allow for MIDI to be passed. That is why VST and VST3 are really the ones to use.
This is a problem with the AU spec and NOT Scaler. Until Apple decides to rectify this, use VST or VST3 versions.