If I store a preset with the EQ switched 'OFF', and subsequently edit it, I find the EQ has snapped back to 'ON'. What's going on?
That's perfectly normal.
When you store a preset with EQ set to 'OFF', the preset is stored as EQ 'OFF'.
However if you go in to "check", the G-System turns the EQ back to 'ON' in order to help you, assuming when you wish to edit the EQ settings, you're like to wish to hear any changes you make - immediately - without having to actually spend time turning it 'ON' first.