There is already a Master Reverb on by default, you can activate it by the traditional method. I would have to check with Aaron, but I believe that AddMasterSubmixEffect adds inline effects, which means that you’ll need to manage dry/wet levels on your Submix Effect Preset and I believe reverb is no dry signal by default because the expectation is that you’ll be using a Send for your reverb (as is traditional for audio production).
