The beast is alive !!!
After 5 days of troubleshooting the non-responsive threshold function (to only found out it was not my error, but only a short in the actual copper traces of the PCB), I finally have a fully working GSSL bus compressor. Only need to install a simple relay system to have a true bypass and I'll be done with this thing. (Of course, cutting the shafts, putting the knobs on and closing the top lid are also in my plans !)