Today I finally replaced some unused coax wires with Cat7 cables and re-added my BeoMaster 6500 / BeoGram CD 7000 combo to my set-up. They are in the attic (study) and connected through a 1611 to go from MCL to ML, and a BLC to go from ML to NL. 
I made a little overview of the whole network :-).
Next trial will be to connect the BL4000s directly to the Core. Until now I kept them in the larger ML network to avoid sync issues, but if they sound better without the Active and BLC in the chain, I will see to fix any delay issues and get an IR eye connected to the Core for Beo4 control in the kitchen. Regards, Johan