Hello- I'm using a Dunlop XL (DVP 1XL) 10k pot expression petal with just a tip sleep connector. When I operate the petal with H9 the parameter values go from say 0 (heel down) to 100 HALF way rocking the petal to 0 again at toe down. The specs on the petal say :Low Friction Band Drive" petal.  Is this not a Linear taper as suggested by you.  Is this a setting issue? HELP 

This problem is caused by using a mono cable with a TRS-type pedal – it has nothing to do with the stompbox.  With a Factor, or an H9 with EXPTIP set to EXP, you could use a stereo TRS cable (as suggested by Mr Dunlop).

If you want to use both a switch and a pedal with H9, you should see https://www.eventideaudio.com/support/downloads/using-both-aux-switch-and-expression-pedal-h9