I noticed the melted wire on 7U also, and I've replaced it completely.
None of the socket terminals are touching and I've wiggled them all with the amp on and no change in behavior.
I've reflowed 12U and pretty much every other joint on the amp with no change.
At this point I've run out of ideas, so I'll most likely be sending it into Bottlehead for them to take a look at. Thanks for the help, though.