Do all SM58's sound the same? I just bought one and tried it out during practice with my band. I also tried a much older, beaten up and rusted SM58, and I could swear the older one sounded sweeter. Do I need to "break in" my mic? Should I have it checked?
Let's answer a question with a question: Do all Fender Telecasters sound the same? Do all Zildjian cymbals sound the same? Do all Fender Twins sound the same? Do all Steinway Model D pianos sound the same? Do all Martin D18 guitars sound the same?
All products change over the years: some for the better, some for the worse. Materials change and materials age. The fundamental design of the SM58 has changed little since 1966, but manufacturing processes have changed, glues have changed, plastics have changed.
Could an older SM58 sound better than a newer one? Yes. But the reverse can also true.
A new SM58 does not need to be "broken in". But its "sound" may change over time. As the diaphragm moves only millionths of an inch, it can be subtly affected by environmental / cleanliness factors.
If you believe your new SM58 is defective, call Shure Service to inquire about having it examined: 800-516-2525.