So, I started out on guitar but now on bass. I see lots of people with bass preamps on their pedal boards and I’m not understanding the purpose. I once used a sans amp di in place of an amp but why is it needed if you already have an amp? Do you leave it on all the time for additional tone coloration? while we are at it, I see lots of compression pedals as well. Leave on all the time as a way to kinda dull the rough edges or spikes in certain frequencies? thanks!