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 },