The headphone amp outputs thru a cable to the headphone socket. It goes thru a switch in the jack and back to the power amp that drives the speakers.
As I recall, and it has been a few years since I worked on a mark 5, that the power amp/power supply was a single sided circuit board. I would inspect it for bad solder connections around the power IC and re-solder as needed.
If the headphone jack is damaged, it may not be passing the audio back to the amp. You could easily remove the cable to and from the headphone jack and jumper the pins to test. There are 5 wires Left out, left return right out, right return and the shield ground. A buzz should be heard when your touch either of the returns with your finger. And if you touch the the sends with the cable connected, they too will buzz Same at the headphone end.
It is not a complex circuit and should be easy to repair.