Just thought I'd follow up on this one.
I tried all that you suggested, and I also tried several other changes. Changed the pot, changed different resistors, tried to implement a trim pot that sent most of the PCB-out to ground before hitting the 3pdt (only worked like an on/off switch, no sound or full sound). Nothing worked.
I've now given up, but I also found out that this possibly is an inherent problem with fuzzes like this. There are several complaints around the web about the Devi Ever US fuzz which is a pedal that sounds very similar to the Factory on high threshold gate levels. Maybe there's something about this kind of circuit that makes the output volume hard or unpredictable to measure for the guys designing the circuit?
I dunno, just nice to know that this may not be my own fault for a change:)
Thanks for your help!