Does a HD621 output MHR in which case, unless that can be defeated, output will not work with non MHR (ie proprietary Meridian) kit.

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