The ernie ball 25k should work correctly with the H9 if you have the cable connected to its out jack. So, the most likely explanation is that you programmed the mix knob to move backwards. An expression pedal mapping has a low and a high setting if you set the low value to mix 100 and the high setting to 0 for example then the mix knob will move backwards as you go from heel to tip. There's more information about this in the section of the H9 Control manual about expression pedal programming (pg 14):
https://s3.amazonaws.com/com.eventide.downloads/Product+Manuals/H9ControlManual.pdf
