Welcome to the forum.

Who told you that 861v4 had no upgrade path? Check out the upgrades Meridian have made available to it only this year... I'm not saying that its ever going to become a v8, but for a product of that age, Meridian like to surprise their users even now...

As for inputs. Yes. All physical inputs and sources are logical associations. You can mix as you wish. Check out phantom sources and if you ever go with a HD621, MHR Smartlink. Apart from limitation of 12 logical sources, Meridian pretty much have flexibility in that department well and truly sussed.

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