new to Audirvana. I am actually Roon-User but I am always trying to convince friends to buy decent stereos and software to make the best of file based audio. Not every of those guys is willing to spend a fortune on a software like Roon. So I recommend Audirvana.
In this case a Pioneer SC-LX901 won’t show as an output device in Audirvana. As far as I know the receiver is capable of AirPlay, UpNP / DNLA, Chromecast…
In Roon the device immediately shows up via Airplay and Chromecast. I can also stream files from a NAS over the network using MinimServer.
There is a regression not fixed by Apple since OS X 10.11 (El Capitan) that makes the AirPlay device unavailable to any application other than iTunes (it is not even listed in Audio Midi Setup application). Until Apple fixes this, there is a workaround to get it back:
Option/alt click on the loudspeaker icon on the menu bar

Select AirPlay in the selection menu that appears
You can then select AirPlay in Audirvana Audio Device Settings.
If the loudspeaker icon is not displayed on the menu bar, you can show it by going to System Preferences, Sounds page, and set the “Show loudness in menu bar” option.
In addition, be sure iTunes is not set to use the Airplay device.
I disconnect Roon from the Pioneer for the sake of argument. No changes. The Pioneer won’t show up in Audirvana. The Pioneer’s network connection via LAN is solid, I double-checked on my router.
Have you checked the firewall settings: In System Preferences > Security and Privacy > Firewall > Firewall Options > Automatically allow signed software to receive incoming connections check that this setting is selected and “Block all incoming connections” is not selected.
Restart your WiFi router because it may have a problem transmitting the connection used by Audirvana Remote (Like the Apple Remote app that controls iTunes, which can be blocked for the same reasons)
I have restarted my router several times (with Audirvana active and inactive)… There is no firewall active on the Mac.
Roon, MacOS and several other apps don’t have any trouble finding and showing the Pioneer. UPnP streaming from NAS (MiniMServer) works flawlessly. But the Pioneer just won’t show in Audirvana.