Masters of what ? Entertaining, singing, music. There may be some who are very good at one or all these components of a "show", they may even be the best in the world but they are not masters. Nobody can master ones voice or ones instrument - that's why they practice, to keep sharp and to get better. I think Barney McKenna is the best there ever was and probably ever will be on the tenor banjo, but he still has to practice. True, all the ones mentioned above may be "MASTERS" compared to others in the same field, but unless you have reached the very top, with NO ROOM left for improvement, you are not a master.