But in theory couldn't a HD621 via firmware update enable bitstream receive and send that bitstream to the 861v9 for proper DTS:X/Atmos decoding just as it can for DTS/DD streams today? The HDMI input switching and audiostream separation layout could then stay the same but now DTS:X and Atmos stream can now be decoded in the 861v9.

I see a place for an updated HD621 for proper 4k with HDCP 2.2 which will be mandatory.

DSP8000SE, DSP7200SEHC, DSP5200SE sides, DSP7200SE rears, 861v8, 210, Sim2 Ultranero 4, Stewart Cinecurve 2.35:1, XEIT CM-5E Anamorphic lens, Dune HD Duo Cinema 8K and Wireworld Platinum 48G HDMI -> UHD722 -> Shunyata Omega Ethernet. Shunyata/Furutech power-cables/filters. SR Master Fuse@861v8. Uptone Etherregen with 10MHz OCXO clock.