I tried the mod as outlined in the CV expander pdf:
An optional jumper position (JMP1) is included on the CV Expander PCB to normal the VCO output to the VCF Audio Input. With this modification and nothing plugged into J6 (VCF Audio In) the VCF processes the VCO signal, but an external signal plugged into this input will break this connection, allowing the processing of just the external signal by the VCF without mixing with the VCO.
To use this feature, cut or remove JMP57 on the Werkstatt-01 BRD-10-011-1111 PCB, and install a wire jumper into the JMP1 position of the BRD-10-011-1112 PCB.
My results are that I get a cleaner signal when plugging into VCF AUD IN, but the LFO seems to be leaking into the VCO stream when nothing is connected to VCF AUD IN.