A HD621 firmware update will not give you full 4k, that requires new hardware in the form of the new 722. I’m amazed the 861 has yet to have an update/new card to give MQA in one form or another.

Main system - Sources (Kodi-HTPC, Vero 4k, FireTV, Sat) into Trinnov Altitude 32 (8-16) (+Roon), Trinnov Amplitude 8 amp & Vivid Audio (Giya G3, V1.5) and REL Stentor III speakers. Others: F80 + Raspberry Pi and other Pi systems.