What I was getting at was : 'DVD' (on MSR) is associated with 800. 'DVD' is not associated with HD621 in any way - ie literally HD621 does not use the DVD (MSR) button so - why is it in the HD621 setup in MConfig?
That's not correct unless you are not interesting in viewing the 800 video output on your screen.
When I had a 800v4 in my system I had:
800v4 HDMI output --> HD621 'H1' HDMI input.
800v4 SmartLink --> MD2 (3x SPDIF)
800v4 DAux --> 861 D3
I then had the Meridian 'CD' and 'DVD' MSR sources setup like this:
CD --- 861 to use input D3 / HD621 to use input H1 (For 2 channel audio)
DVD - 861 to use input MD2 / HD621 to use input H1 (For 5.1 audio)
This way when CD or DVD is selected on the MSR the HD621 should switch to that HDMI input and I could see the 800 on screen configuration menus and DVD / DVD-A menus / images on my TV.
If "Last Valid" is grey out on the HD621 this suggest you have not configure in the 800 HDMI output as a source into the HD621.
I'll email over a Mconfig file for you to have a look at ... it's for my system back in 2013 with a 861/800/HD621 and DSPs (though it's 861v6 with the MMHR link but it could still be a useful reference for you).
Regards,
Carl