Recently I rounded off the head of a tube nut. Tried various ways of removing it, none worked.
I found a tool that works great at removing rounded heads. Vice-Grip wrench: Not vice-grip pliers. This tool has 3 sided jaws that tightly grip on the remaining sides of a rounded nut. It grips so tight that even when pulling very hard it did not slip. The nut came off. Has the advantage of being usable on tubing nuts where box end wrenches cant be used. I found mine at Westlake Hardware store.