If you want higher resolution then from IOS you will need to use a wired connection from the portable device to the DAC, potentially via a camera connector. The reason I say potentially is because Lighting is not fully USB compliant and some DACs will work and some won’t (just depends how sensitive the DAC is to adherence to the USB standard), but the camera connector takes your lighting output and makes it USB compatible so any DAC with a USB input should work.