Wow, I’m supprised. I don’t think I’ve ever owned a pedal that does not have an output level control. So let me get this straight, if I want to control the volume of a preset I’m going to have to open the app on my phone?
I don’t wish to sound negative. The H9 is a great tool and I’m very pleased with it.
The original question was whether or not Output could be assigned to X, Y, Z. If you wish to set output level for a preset:
Press and hold the Left Footswitch – the Preset will now be Active. After holding for 2 seconds, the display will show [OUTLVL], and then a second later  (or the current value). Continue holding the Left Footswitch while turning the Knob to adjust output level from -12 dB [OUT-12] to +12 dB [OUT+12] in 1 dB increments. Release the Footswitch – the Preset will return to its previous Active or Bypassed state. The next time you do a Press and hold of the Left switch it will return to the last thing you adjusted.
Note: Preset output levels must be saved even if there are no other changes to the preset or the output level will revert to its previous setting the next time the Preset is loaded. In other words, if you want the adjusted output level to ‘stick’, you must re-save the Preset.