I suspect the 861 v9 or 10, will not have any HDMI inputs or outputs.
The video is removed at the HD621 and only Audio passed via MHR to the 861.

Logically there will need to be an updated HD621 which supports HDMI2.0a for full 4K and HDR/Dolby Vision pass-through to the output video device?

No doubt the new 861 will support MQA.

As for the new surround formats, and the extra channels required (at least 2); the HD621 does not decode bitstream, and only processes PCM?
DTS:X and Atmos are only contained in the bitstream.

So the HD621 will have to decode the DTS:X /Atmos and pass it as extra PCM channels via MMHR to the 861?


861v4 LPSU, HD621, Sony HW40, PMC AML2, PMC IB2SA Centre - Triamped via Wyred4Sound MMC, PMC Wafer2 surr, PMC SB100 Sub & Wyred4Sound SX-1000, Dune HD MAX. Oppo BDP-105D Signature. Mede8ER 600X3D
Marantz CD94/CDA94. Rotel RB-1562, LG 65" OLED. IPL Acoustics STL5. Virgin Tivo. HifiMan HE-1000/Audio-GD M9. Stax SR-007/KGSSHV Carbon‏.