Great question. Let me start with a quick high level overview: When you play a Blu-Ray with TrueHD or DTS-HD MA, the Blu-Ray player sends that format to your receiver, and the receiver decodes the audio before sending out to your speakers. LPCM is just another Lossless (HD) audio source that can be directly streamed to your receiver without decoding.
Unless your TV has eARC (which is still very new), it will only be able to pass 5.1 lossy audio down to your receiver.
The Apple TV supports up to 7.1 LPCM audio. Infuse is able to Direct Play TrueHD and DTS-HD MA by decoding the audio to LPCM in-app, which the ATV can then send to the receiver for playback.
The only downside is if you have an Atmos or DTS:X stream, you will loose the spacial information when decoding to LPCM and be left with just regular (lossless) 7.1. There is a solution to that with Dolby MAT 2.0, but that is a road block Apple has to clear, and a discussion for another time.
The Nvidia Shield is able to direct play TrueHD/DTS-HD MA in Plex because the Nvidia shield natively supports those audio formats. So if you need Atmos, definitely get the Shield. If you just need 7.1, Infuse will do just what you need on the Apple TV.
HD audio support is only available with Infuse Pro because they have to pay licensing to Dolby/DTS in order to decode the audio. I bought the stand-alone Infuse Pro app instead of the lifetime subscription because I am holding out for Plex to have the same capability in their app.
Infuse also has some rather robust subtitle support.
