Clean setting: gain down
Saturated setting: gain up
The voicing knob adjusts the EQ curve with some presets for type of instrument/vocal. Play around with different EQ settings to see what you like (with these things, the EQ for the specific instrument is rarely actually the best for the instrument) - some will add more saturation than others.
It really depends on what instrument you're using, though. If you're using an electric guitar, bear in mind this isn't a preamp designed specifically for guitar... so you're not gonna get the gain levels as you would from a preamp designed to overdrive a guitar. So, if you go for break-up (distortion), you're probably not gonna get what you want. But you could very well get a nice clean-ish saturated sound (cranking might get an interesting sound, but probably not quite what most people are going for).
But yeah... for clean, gain = down, output volume = up (or, ya know, at least at 0). For saturated, gain = up, output volume = down (to even out the volume, obviously). Pretty straightforward.