I have a similar but not exactly the same problem with my Spider V30 MkII.
I can not easily switch between "Headphone" and "Line out" modes. Even when "Line out" mode is set up no sound is heard from the speaker when Spider is switched on, i.e. it is working in the "Headphone" mode. To switch into the "Line out" mode I have to unplug the jack from the socket, go into the "Settings" menu, switch to the "Headphone mode", then switch back to the "Line out" mode and only then plug the jack back into the socket. That is disturbing as unplugging and plugging jacks right on the stage can produce unnecessary and loud pops and clicks.
Something is not very good with the "Phones/Line out" socket.