sorry to bring this back up again.
Can I now clarify, with the instructions that are included with the 57 and 58, that the frequency response is exactly the same. Now, if you were to take the ball of the sm58 and place the mic in an identical position as you would a 57, on ANY instrument for that matter, you would get the same sound, give or take that every mic is not EXACTLY the same, i.e. 2 x 57's made on exactly the same day could sound ever so slightly different. Now, some say the grill left on a 57 would make a difference compared to a 58 without the ball, I say complete rot. The difference with having the grill on a 57 and not on a 58 would only effect protection to the mic.