Gentlemen, I think I have resolved my issue and as usual it is an operator error.

I went back to the configuration Marc Koval had installed when he repaired the 861.

Via the print out of the configuration, I discovered that I had failed to connect the HD621 to the Meridian comms via Card: C000
I only had the HD621 connected to MD2 and maybe that is why the center channel and sub would not play periodically

I made the connection from the HD621 (c) to the 861 C:000 and now the center and subs are outputting.

I will keep you posted if this is not the fix.

regards

Samuel

861v8, HD621, LG oled, Mac Mini M4, Gustard U18, Apple 4K TV, B&W 801's Matrix 3, B&W HTM1 & HTM1, (4) B&W CCM80's (ceiling surrounds), (2) Velodyne HGS18 & Keces 3600, Furman Elite 15, (2) CineNova EarthQuake 5 channel