I'm 95% sure you'll have no problem just setting your AirPlay device as the DAC in Audirvana. Now, AirPlay is the protocol… we need more information as to what is receiving the AirPlay stream exactly… AirPort Express? Apple TV? Either of those you can just run an optical cable (TOSLINK for TV, TOSLINK mini (1/8") for APE, I believe) to LIO. Either of those solutions are digital the whole way through, neither the Mac's DAC nor the DAC in APE (TV has no DAC that I'm aware of) would be used. I do not believe there is a USB solution for either of these. APE's USB port is for printer sharing, possibly storage, but I'm quite certain there's no USB audio support. If you're connecting a computer to LIO via USB, you are correct there as well — the only DAC in play will be LIO's. I know you're planning to use Audirvana, but know that iTunes supports bitperfect playback as well without issue.