Check out https://rogueamoeba.com/airfoil/ which can broadcast any audio on your computer to any airplay device.
You should be able to broadcast anything to your airDAC that way without selecting it as your wifi network. Make sure your airDAC is properly configured to join your wireless network and not to broadcast it's own network. When your Benchmark comes in, I'd try using the Coax out on the airDAC to feed the Benchmark - since you already have it and it would probably result in the least dropouts.
I use Airfoil to airport expresses in my house and it works great, but have found that the airport expresses only works with some DACs. Some work great, and some can't maintain a lock on the optical signal. By using the airDAC as the airplay receiver I'd expect this to work reliably.