The G91/92 does not have (discreet) multi channel digital input so you could not use the HD621 except in 2 channel mode.

I initially intended to get a G91 to replace my G68/G98 combo, but lack of multi channel digital input stopped me.

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).