Hi frog,
There is also an other solution: Datalink to ML converter.
I have this solution in use, it is a small box called Beolink converter, see here:
http://www.beoworld.org/prod_details.asp?pid=983
Connect cable 7 pol DIN Datalink from the Aux or Tape2 socket Beomaster 5500 to the Beolink converter, and than connect your ML into this converter for linking to the rest of your Beolink network.
MCL/ML converter has the advantage to have a amplifier and is intended for MCL Beolink wired house to newer ML Main products.
Regards, Robert