I am mainly suspecting the tube which handles the effects loop. It's usually V3, but you will need to check your manual to find the correct tube. My thinking is the headphone output would not come from the power amp but the preamp instead. So if the problem occurs yet the headphones remain with sound then that means the problem is power amp. What I would expect instead is that both headphones and cabinet behave in the same way. For an attempt to fix without diagnosis, just swap out the V3 tube (again check me against your manual) and see if that helps. If you don't know which one it is, feel free to swap out all with your backup set, but keep in mind which one went where.