I don't find it to hard to get my head around. Drive off, it's just like any amp, gain doesn't have any real effect on the sound. Dialing in drive lowers the headroom, so that the gain control starts to produce soft clipping overdrive.
It also starts to roll off highs and lows as you turn it up, so you can think of it as a modern/vintage knob. At low gain, you could just use this drive control as a HPF+LPF speaker simulator.
The final thing to realise is that the mid control is placed before the clipping section, so has a massive effect on the overdrive character. Boosting mids gives you more drive, but boosting bass/treble does not, since those controls are placed after the clipping section. Therefore don't be afraid to drastically scoop mids if using lots of drive!