Regarding the schematic, another user posted the XTRA manual links for you that shows how to hook things up.
I have a Beomaster 4500 system (which is MCL) connected to a Beolink which connects via ML to an Avant (which is ML) connected to Penta 1's and Beolab 4's in the rear, with the XTRA/7000 in another room. This is the same setup you could do, minus the MCP 9000.