Set the gain knob (red Marconi) around 20 on the dial and set the output (grey knob) fully clockwise. This gives you the highest headroom and cleanest signal path. Every click is in 5db increments so if one click is too hot, attenuate the output to get inbetween the 5db steps.

Another trick people like to do is to put the final mix through the line inputs on the back to get that harmonic distortion. To get even more of that Class A output stage is to come out of channel 1 and into channel 2 then you have an accumulative effect of that classic output stage.