I failed to recall somehow years ago when I used to connect a Samson Graphite MF8 to PIANOTEQ and had ductile control at the MIDI hardware to quickly send MIDI events to one of the EFFECTS (whether my personal mapping switched on or off the parameter I had just previously assigned via Global MIDI Mapping).  Which Global MIDI Mapping under Options and per your personal application of whatever MIDI assignments you (yourself) stipulate permits hardware complete control of software slots 1, 2 and 3, even to include specific slot parameters like Mix, Duration, Room Size, Etc.  All of which are controllable from a MIDI controller.

janahf, you have really to become familiar with the Options menu, specifically the MIDI tab within it.

There under Global MIDI Mapping you perhaps need firstly to press the Insert button and secondly the parameter name in the fourth column of the uppermost row that appears when you've pressed that button.

Now, whenever you option or alt click onto that (Volume) you get a menu which itself reveals Set Parameter just as hesitz believes:

Could not find this topic via search, so apologies if its been answered before.

Is it possible to assign a single MIDI button to toggle one of the rack effects on/off? The little green "button" that toggles the effects on/off in the application does not appear to be MIDI assignable. Am I missing something?

Isn't that little green button exactly what's controlled by the Effect 1 > Switch, Effect 2 > Switch, or Effect 3 > Switch that you find under the 'Set Parameter' menu?  They aren't named because you can have different effects loaded into slots 1, 2, and 3.  But I believe that Parameter is what you modify to toggle the effect slots on and off.

You may just have to create a whole new preset that you can assign later to a program change via whatever interface (MIDI hardware) you use.

thanks for your reply, amen. it’s certainly possible to set this up (crudely) in the FX preset manager. assign one midi button to “none” and another button to the preset you’d like to recall.

however, this doesn’t work super well, as it doesn’t save any FX parameter adjusting you’ve done once you switch the effect off. seems very strange that this would not be a MIDI assignable event? i really feel like i’m just overlooking something!

Is it possible to assign a single MIDI button to toggle one of the rack effects on/off? The little green "button" that toggles the effects on/off in the application does not appear to be MIDI assignable. Am I missing something?