Keeping the Master Volume low is the wrong advice. Relevant is the respective output volume. Master Volume could be low and one individual output volume high. Relevant in this case: Monitor Output Volume.
This is from the Main Manual:
The internal power amp is mono and delivers 600 watts at 8 ohms; however, the wattage will be lowered to 300 watts when used at 16 ohms. The power amplifier provides sufficient headroom to avoid clipping, even with dynamic clean sounds. Very loud signals and transients will then be compressed by a nice, organic-sounding soft-clipping circuit. You should, however, use these 600 watts of power with caution - if you are running at high volumes and notice any indications of speaker distress, turn down "Monitor Volume" or "Power Amp Boost".
Make sure monitor volume is linked to your master output, then make sure that’s right down to start with. The volume of the power amp is also the same as the monitor output. They are basically the same thing.
I think that’s the obvious place to start, as you’d already thought.
There are 2 factors here:
General volume
Managing peaks/accidentally sending too much signal
For general volume, you are unlikely to damage a 4x12, your ears will bleed before you blow the speakers. So although its a 600 watt amp, its so it has plenty of clean headroom. I have run a regular 4 x12 rated at 150 watts no problem, and I've played loud.
For the overall volume management, advice above 
