As Shadorne says, you adjust it so that your preamp's volume control has the most range. If at 32dB, you only need to turn your volume 10% to make it loud, then changing to 26dB will allow you to turn the volume to say, 30% to make it loud. You should set it to whatever you find better. You may not need lower gain (although all things being equal, 32dB is a lot for an amp).
Arthur
Arthur