Audirvana can play to UPnP endpoints, but UPnP is often not very stable/reliable. For my headphone amp, I use a squeezelite endpoint (PiCore Player) connected to my DAC by USB. I would very much like to be able to play to this endpoint from Audirvana. LMS is much more reliable than UPnP and easy to implement. The Slimproto protocol that would discover and connect to squeezelite clients is well documented and open source.