For headphone preview/pre-cue in iOS your audio interface needs 4-channels (4 out). This will show up in djay Pro as stereo channels (1-2) and (3-4).
Examples of 4-channel (4 out) audio interfaces: MOTU M4, Audient ID14, SSL 2+ (Plus!), Presonus 26C and Scarlett 2i4.
Tip: iOS uses the last audio interface that was plugged in. So plug in your DJ controller 1st and the audio interface 2nd. Your DJ controller won’t show up as an audio interface but you can still use it to control djay Pro.
Notes: In macOS you can route the main audio (1-2) and headphone audio (3-4) to different devices; but iOS can only handle one audio device at a time - so you need a single 4-channel device to preview/pre-cue in iOS.
