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.