All USB-C iPads lack a headphone jack. So the sound quality of these devices is solely dependent on the DAC or audio interface you use. If you use an audio interface, you will get identical audio quality no matter if you connect it to your iPad mini or the new iPad Pro.
I am very happy with the audio quality of the headphone jack on Apple devices. It is very high compared to for example Android tablets but running through a dedicated audio interface or directly into a digital mixer via usb will always win in regards to audio quality. The question is: can you hear a difference over a PA in an untreated room? I can't.