I think you should be able to get a HD621 significantly cheaper than £750 - there was a recent trade in on them so a bit of a glut of them from official sources.

Hopefully that would allow you to put the difference towards a 861v4 to narrow the gap.

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