Thank you very much, Madskp.
Yes, it looks much easier to find an Beolink Converter type 1611 than a ML powerbox (at least here in Switzerland).
If I understand you correctly, I would connect the Beolink Converter type 1611 with a Masterlink cable to the NL/ML converter. But how to connect the NL/ML converter to the rest of my Masterlink setup (3 Beolab 3500)? There is only one Masterlink socket on the NL/ML converter.