Thanks @codamedia and @Kilrahi. This is the first Helix product I've had so I'm really just still getting to grips with it... I actually tried using the Aux In 1/4" jack before I posted (just trying to get a mono signal out of the L channel to get started) but didn't get any audio through into my IEMs... Your responses made me go back and have another go. After some changes in Global Settings (Ins/Outs page) making sure that "Send/Return L" and "Send/Return R" = Line level and "Return Type" = Aux In (this was set to Return before), I now have it working correctly, so I'm made up. Now to get a Y lead 3.5mm stereo to 1/4" mono, like the one @Kilrahi linked to above. Even better that you can stream digital signal eg a Youtube clip into the Stomp and have that coming into the mix on the IEMs too! Thanks guys.