I have an HDFury, allowing me to route audio to HD621 and picture in 4k to TV, as the HD621 doesn't do the 4k for me (it then gives lots of artifacts).

drawbacks:
- losing the HDMI switch capability of the HD621
- losing lipsync

So, although it is not pretty, it does the trick for me.

Meridian G68 AXV, HD621 | Cinepro 2k6mk5 | 4x Scanspeak A4 monitors, center & 26W Sub | RPi Hifiberry digi+ | Roon