Prevent headphone line-out muting amp speakers (Vox Pathfinder 10B)

Make headphone jack non-muting?

This was discussed already for this specific amp in the above link but I'm asking specifically regarding my amp. The main speaker is disabled almost the moment I start putting the jack into the line-out socket, so I'm unsure if it's a physical switch or the jack itself is closing the circuit.

I don't know if the schematics are available online but if anyone knows how I can explore this, please share! e.g. tests I can do.

I took the back off and the jack socket has 3 'rings' each sprung in turn as the jack is inserted, so it has a triple hold on the jack. The jack socket is mounted on the main circuit board and I can't see underneath how it's wired up.

Thanks.