Music library is on iMac (High Sierra). Stereo rig is on the other side of the house, beyond the reach of Ethernet. Thus, wireless is the setup. An AirPort extreme is attached to a DAC (Marantz 7004) by ethernet. So the iMac transmits over wifi to the airport extreme, and that is connected to the DAC with a one-foot ethernet cable. With this setup is there anyway to use Audirvana Plus? The FAQ and user manual don’t address this configuration. If I instruct iTunes to not connect to AirPlay, my only option is built-in speakers. Which is an iMac. This is so baffling! Without A+, the streaming rig works great. But the whole point of A+ is to make great turn into awesome, and I’m not there, dang it. Any thoughts appreciated!
Yes, you can use your remote DAC since it’s available to the iMac over AirPlay. Select the Airport Extreme in your System Preferences: Sound as your sound output device, and then, when you launch Audirvana, an AirPlay option will appear in the A+ Audio System Preferences. Select that and confirm your remote AE to make it happen.
Another option would be to set up a UPnP/DLNA client at the DAC location using new hardware and software, which could be as simple as a $35 Raspberry Pi running a free OS like DietPi. -But now you’ve jumped in the Deep End.
But using UPnP from Audirvana+ to a Raspberry Pi 3 B running Moode Audio (e.g. 4.0 beta) works like a charm, wired or wireless.
Using A+ Remote from my iPad, navigating either the Mac library or Tidal 
Now my Dac Magic Plus is in the living room - I’ll have to get something else for the office 