If your Mustang I-IV has the newest firmware (1.10 is current at this writing) you can hold down the EXIT button and turn up the Channel Volume knob (NOT the MASTER volume) and this will adjust the USB output volume. Works great in Garageband.
NOTE: The volume change is temporary, and it is not global. It puts the current patch into edit mode, so if you switch patches, you'll have to reset the volume. Hope this helps!