Bernard, we've been waiting breathlessly to learn if you're plugged in to the right output. If that isn't it, a question about check 2. : do you mean that you measured a voltage output at the end of the cable and that "nothing" referred to sub sound rather than voltage? If so, how much voltage?
A remote possibility is that the sub amp had a sudden failure. Check that by disconnecting one of your main speaker wires and temporarily connecting it to one of the sub's speaker level inputs to check for sound.
