I will add the trim setting very soon. I don't really recommend this, but at the moment you can sort of hack your way to doing that by opening the settings file at "~/Library/Preferences/MiniMeters/settings.json" and adding or changing the line that says "trim": 0 to the decibel value you'd like to gain the signal by. The modified audio section should look like this:
"audio": {
"buffer_size": 4096,
"channel_left": 0,
"channel_right": 1,
"device": "Desktop Audio",
"trim": 0
},