I've been using my Line 6 HX Stomp with the M-Audio with a Y stereo to mono splitter jack. On the tip I have another stomp pedal for triggering the tuner (set as FS4), and on the ring I have the M-Audio EX-P (set as EXP 2). This way the wiring works fine.
My problem was with getting full range. When using a Wah it's OK as it's not perceptible, but when using a Whammy/Pitch shifter the minimum position was always floating from 0% to 5%, instead staying fixed on 0%. This distorts the original pitch, which is very annoying! So I came here, found this amazing conversation and saw the resistor suggestion. In my case, I realised that shorting the resistor would give the opposite result, it'd increase the minimum position (floating from 5% to 10%), so I removed the original resistor (1K) and soldered a 4.7K in its place. With more resistance now it works perfectly!!! Phew, I almost bought another expression pedal because of this. Thanks guys, hope my solution can help others as well.