Try tweaking the MIDI CC value range on the receiving end (in the H9 Control app). I just send a full 0-127 switch action to Parameter 8, which would be SCALE in the QuadraVox algorithm. I kept the Parameter Range Start point at 0, and reduced the End point to ~10. Maj to min scale switch. You have to experiment with the Start & End values, as they have a mismatched range of 0-100.
There are at least 4-5 main ways to switch parameters like this, along with combined variations. MIDI being the most versatile, and also more complicated. I have all the MIDI CC value ranges for KEY & SCALE posted here on the forum. Hell, with a MIDI ‘board relay switch – and a hack / mod to the SuperEgo – it’s even possible to freeze the SuperEgo & change KEY / SCALE from the same MIDI switch.
Direct Aux switching will work, but those Parameter 1-10 settings translate differently across algorithms and sub-types. And yes, preset switching is the most basic, yet still functional approach. It’s the same algorithm here, so only a single parameter change gets refreshed. If only one SCALE change is needed, the Preset Range can be set to 2. Effectively, toggling between two scales, with a latching action.
If you want both an expression pedal and a (single) Aux switch, you need a Y-cable; merging both devices to the input jack. The expression pedal alone can also ‘morph’ among multiple parameter ranges. And, for an alternate approach, with no expression pedal, and (up to three) Aux switches:
Bind an Aux switch to the expression pedal programming. ‘Set expression pedal value (aka HotKnob) value‘ allows you to program any combination of parameter ranges, and save those settings per-preset. Then the mapped Aux switch will alternate between the two pre-programmed extremes. An Aux switch that’s selectable between momentary & latching action is the most versatile for this.