As pointed out, there is a Global Setting to Mute the Outputs & Headphones. If this does not work, the DAC (Digital to Analogue Converter) is the only component that the Outputs & Headphone have in common, but that the speakers do not use. This component is IC807 - AK4396 surface mount component on the DMM PCB. Before the HP DAC, the digital signal splits off to a separate Speaker DAC (still working); after the HP DAC, the signal splits to two separate Op-Amps - one for Headphones and the other for the Outputs (neither working). Good luck.
I will give it a careful approach. This should work as its so close to my assumption but i didnt want to attempt first until i see a clarification on it.