I have now solved the problem by setting up a control switch as suggested. I noticed, however, that the effect blocks are bypassed by default when the footswitch to which I have assigned the CS lights up, and the blocks are activated then the light is off. Is this 'standard' behavior or can I change the 'direction' of the CS? As a workaround, I have set the start value of the modifiers to 100% and the end value to 0%, which results in a negative slope of the modifier curve and effectively swaps the action of the switch.