H9 has a FAQ page where it's clarified:
Our stompboxes will work with any expression pedal that is a simple resistive load (potentiometer) with a max value of 25k Ohms. For the best performance we recommend the Ernie Ball VP Jr. 25k. Connect the output to the Eventide stompbox expression pedal input. We have also have heard from users who had success with the M-Audio MGear EX-P, Line-6 EX-1, Mission Engineering SP1-LF, Source Audio Reflex Universal Expression Controller, Roland EV-5, and Boss FV500L.
New for H9:
Mission Engineering has released an expression pedal specifically designed to work with the H9. The SP-H9 has a potentiometer wired as a rheostat and an integrated momentary switch connected to the ring. This allows the customer to connect the expression pedal to the H9 with a single TRS cable and get both expression pedal and switch functionality in the one unit. There is also a parallel switch input so customers can connect an external TS switch just using a regular 1/4” instrument cable. Learn more at: http://missionengineering.com/?product=sp-h9-eventide
If your expression pedal has a trim pot control, try adjusting it to either end of its range. If your expression pedal causes parameter values to go from min to max and back down to min again as you move from toe to heel, try connecting it using a stereo phone cable rather than a mono cable. If the calibration seems wrong, try plugging the expression pedal in before powering up the stompbox.
H9 has an expresion pedal calibration feature. You can try turning it on and moving your pedal from heel to toe a couple of times and then turning it off to calibrate your expression pedal.
Here has more info about expresion pedal if you want to use both an Aux switch and an Expression pedal:
https://www.eventideaudio.com/support/downloads/using-both-aux-switch-and-expression-pedal-h9
