I've been trying to figure it all out today. Documentation seems scant. It looks like HDX just becomes a glorified playback card for the Dolby Atmos Render. Pro Tools -> Atmos audio bridge -> Dolby Atmos Render -> HDX -> Speakers.
On one upside, it increases the voice count in Pro Tools to 384... but it does make a HDX card a little useless indeed. When using the Dolby Atmos Audio Bridge, it appears the render expects some kind of sync to function as well. Either MTC or LTC over audio, where as the send/receive plugins handle it all internally. I had to generate LTC audio online and stick it into the session, outputting it to the Render on channel 129. It seems to work fine after that.
HDX seems to be on the way out...
