By default, higher notes will sound louder than lower notes coming out of your synth, even when played at the same velocity. Because of the way our ears work.

If you want the lower notes and the higher notes to sound like they are the same volume, you need to apply some kind of modulation to the gain based on the note number.

I'm surprised it took me so long to realize this.

I wonder if there are some synths that do this by default?