Just picked up a Bartolini Piezo buffer (AGPB-918) and, like every other preamp they've made, it comes with a "trim pot". My questions for those who have experience with the trim pot is: does this act as a volume? Is out of entirely out the way when on 0? My two magnetic pickups both have their own volume and tone pots-no selector at all; so, should I give this its own as well?