In case this helps, I have my humple experience with the following EXP pedals in conjunction to the H9 in my home studio:
- Electro Harmonix Expression (the “cradle” like one, where your groundfloor is the base): works flawlessly and is much fun; this is my EXP of choice. Very smooth transition and responsible control, IMHO.
- BOSS EV-30: works transparently, providing you find out the right settings on the pedal.
- BOSS EV-500L: same as above, although somehow oversized. On the other hand, the size gives you even more precision in control. So if a kind of Laurel-and-Hardy-look isn’t an issue for you, this giant dino may be an option. I’d rather use it to control my Matrixbrute.
- Thomann Lead Foot (a 19 Euro cheap piece): unstable, likes to still send random signals even in the heel position, which makes the display go crazy.
Unless stated otherwise, the pedals mentioned above worked for me with H9 solo (connected via TRS) as well as in conjunction with BOSS FS-5U Switch (both connected by Y-cable 2x mono -> 1x TRS)
