The line out feature is a bit unintuitive, but you have to turn line out on, then while the volume screen is up (turn the volume knob), you need to press the small “line out” button at the top of the screen. It can take a few tries, but that’ll set it to max volume.

Just registered here to say that AK Connect in the latest update is a total game changer for this thing! I’ve been using mine plugged into my hifi for the past couple of weeks and being able to control it from my phone is perfect. Thank you so much for making such a great device, @JasonNYC!