So I've done some testing
As @peter says the audio out of headphone out of a 3.5mm adapter is very good.
The internal DAC (digital to analogue converter) in an iPad/iPhone is a chip on a circuit board.
If you have a digital mixer, I would encourage you to use lightning to USB in on the mixer
This eliminates the digital to analogue conversion.
So basically your audio file is passed to your input channels with no loss,
If your audio is wav it retains it all, if it's 320 MP3 it retains it fully.
Also iPad/iPhone compared to headphone out of a MacBook is noticeable also, there is more fidelity and dynamics retained using the MacBook headphone out vs iPad/iPhone,
This is probably due to the MacBook have a dedicated soundcard and not a chip on circuit board.
So in short, try to keep your conversions to a minimal or at all if possible.