low vs high gain

Whatever works best with the preamp (allows you to play softly and loudly).

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

Keep in mind that using the higher 32 dB gain setting does not increase the power output of your amp. The max is the max no matter which you choose.

As noted in a previous response, the switch is simply there to give you more flexibility in matching to your preamp. I'd recommend turning your preamp volume knob about half way up and then choose the amp gain setting that gets you closest to your desired average listening level. If the 32 dB setting is way loud, use the lower amp gain. That will allow your preamp volume knob to give you the best volume fine-tuning in it's "sweet spot."

My guess is the higher gain setting is primarily for passive preamp situations.