I have a JTV-89 and have tried plugging it into a Helix LT, Pod XT 3 Live, as well as the Variax AB box and power supply with a TRS cable. In none of these instances have I been able to use the onboard knobs to control the guitar model or tuning, the guitar apparently reverting to the magnetic pickups. I have gone into the device and turned off forced modeling with no change. Oddly enough, everything worked fine with my old HD500, which I traded in to get the Helix LT, which doesn't seem to do the trick.
Any idea of what I might be doing wrong?
