I've had this unit for a month, so here's my non-audiophile thoughts.
I'm using Hifiman Arya Organics with this through the balanced connector. Sound quality was tested with both Spotify, Apple Music, and Tidal. The unit is able to stream Spotify, Tidal, and Roon through a wired network connection only.
The EF500 sounds great compared with a Fiio K7. Fiio was a bit “shoutier,” and the EF500 is smoother overall, and tames the higher frequencies without recessing them. Paired with the Organics, I don't feel the need to keep looking for better-sounding, more expensive equipment any longer. Also, I tend to favor OS mode over NOS mode with the testing I've done.
The EF500 can't be used as just a headphone amp with a different DAC than the one that is included. There are two USB inputs, along with a coaxial input, but no SPDIF - so it might be tough to use this with a TV as the source without a signal converter of some sort.
The problem I had with the EF500 is the streaming. Direct connection over USB to both Android and Apple phones worked flawlessly, but the ball has been dropped regarding network streaming. Using Spotify Connect works, but the first second of every track is cut off with silence, no matter if an album is playing sequentially, or if random playback of a playlist is used. Communication with Hifiman support wasn't very useful - they claimed that they couldn't reproduce the problem and they blamed my network. Meanwhile, I have a Minidsp SHD hooked up to the same network switch, and don't have seconds missing from any song at all using Spotify Connect on that, ever.
Tidal Connect, on the other hand, while *occasionally* the first second was cut off, it usually played the entire track correctly. The issue with Tidal is that the app would lose synchronization with the EF500, and cease to control streaming whatsoever. Exiting completely out of the app and restarting it is the only way to regain control. Finally, I couldn't test Roon as I don't use that application.
So, if you plan to use the EF500 with direct wired connections to the source, I definitely recommend buying it. But if you're going to have it set up far away from where your phone is, I might think twice. I asked Hifiman support if any firmware updates are forthcoming, but they didn't indicate that would be the case.
Sound - five stars
Streaming - three stars