There are litterally 1000s of heavy metal presets available through the app. It seems that the app will only show a maximum of 30 results though, so be specific when searching and try different search parameters.
It seems a lot of people are confused about the "Amp Out" on the Amplifi (can't blame the manual though, it is quite clearly explained there). If in doubt, use the Main Out. The Amp Out is used when you want to separate the guitar signal from the streaming music. For example to send the guitar signal to your guitar amp and the streaming music directly to the PA.