In this article you'll learn how to adjust a MainStage sound's velocity. This is helpful if you'd like to make a sound quiet when playing really lightly or softly and you don't want to have to adjust the volume of that sound.
There are a number of ways to do it, but this article covers the simplest way.
First, you’ll want to put the MIDI effect “Velocity Processor” on the channel strip, then bring the “Make Up” knob down to about -45 or so. This MIDI effect basically works the same as a compressor, but instead of compressing the audio level, it compresses velocities (which is pretty cool!) You can play around with the settings a bit to make sure the higher velocities still respond the way you'd like.
This won’t make the sound (like a piano for example) completely silent, but it will bring it down to about -50dB when played at a velocity of about 40 or lower, which (especially when layered in with other sounds) is basically inaudible.