to keep this thread alive :)
I can confirm that the Dunlop Volume X (DVP3) pedal works with a Helix.
You need:
A standard instrument lead (NOT TRS)
To switch the blue switch inside the pedal (this reverses the toe up/down on the pedal)
Invert the EXP 2 pedal in global settings on the helix
If you don't switch the switch inside, the pedal exhibits the 0-100-0 behaviour reported by others when using a standard instrument cable.
hope this helps someone......