Hi david

> only a few modules work correctly, a few not at all and a few with a very bad ground hum.

Did you measure both voltages with one of the "bad" modules plugged in

Did you check the modules separately one after the other, or did you
plug in all at the same time If the latterm then try them separately.

I do not know the non-Doepfer modules you mentioned, but maybe they rely
on a connection frontplate -> electrical-ground (0Volts).

Florian