Ok, the solution is to create an Aggregate Device…
- On your Mac open Applications > Utilities > Audio MIDI Setup
- In the bottom left click the + and choose “Create Aggregate Device”
- Rename this device to or leave it called ‘Aggregate Device’
- Choose the Mustang LT 25 (or other USB device) for your Input
- Choose an Output
- In Cubase open Studio > Studio Setup > Audio System > ASIO Driver
- Choose or ‘Aggregate Device’
