I use an 128gb 10" iPad as a source with my external standalone DAC. Just need the right cable and app that will output the digital bits via the cable.
It can output digital bit stream via the Lightning to USB cable (the one made for Midi, or if you need to also charge your iPad during playback, the lightning to usb camera adapter is the one you want) and passes along hires digital signals up to the DAC maximum (512 DSD, 24/192 PCM, DoP, whatever). The onkyo hd $5 paid version of the app from the app store can do this, and the standalone DAC does the digital to analog conversion using that data input via USB in digital bitstream. Screen on DAC confirms it's receiving a digital stream and displays the bitrate and type (i.e. DSD 512/128/64/etc, PCM 24/192/etc, 44.1 WAV etc, etc.,....)
I use the onkyo HD app, but it's pretty old, doesn't get new feature updates, and I'm sure there are newer/better apps for iOS that can transmit hi-res digital signals.