The H9 only needs two values to drive the expression pedal. My colleague, Nick, might chime in to clarify which of the two values are needed of the three: tip, ring, and sleeve, and which two values a mono cable would likely be sending. But it's likely that a mono cable isn't sending the two values expected or not in the expected order. The manual of the 500-L says to use a stereo cable: http://www.sweetwater.com/store/media/fv500l_manual.pdf
