I believe the only pedal that will work for EXP 2 with the POD HD 500X, is the Line 6 Mission EP1-L6 pedal.
No Calibration. I use EXP 2 as a Volume Pedal. EXP 1 is my expression pedal for WAH (EXP TOE SWTICH= ON), or other Pedals for Mix, etc.
If I have maxed out on DSP, I will use EXP 2 to control the AMP CHANNEL volume. All this is done in Controller.
dH