I have a newer VOX AC50cp2. I have the exact same issue. So coming across this thread has me thinking it may be the internal circuitry of he VOX itself. The effects loop of the amp works normally with just a standard setup, but with the pod 4cm it seems to just run through the amps preamp. I've tried all the tests too and still find the 4cm just doesn't work with my amp. It's not terrible, but now my curiosity of what this pedal can still do has me climbing the walls.