Having read this article
I bought a wee gizmo that is a male/female XLR connector with a resistor soldered across a couple of pins.
The theory is that the SM57 and 58 were designed for use with desks with an input impedance of about 500 Ohms, whereas most modern desks and preamps are 1500 to 2500 Ohms. The gizmo changes the impedance that the mic "sees" to 500 Ohms. The result?
I did a quick voice test with my 58 into a RNP pre and Pro Tools, comparing the mic with and without the gizmo. The gizmo cleans up the mud and reduces sibilance. It's a cleaner, clearer, richer sound. Much nicer to my ears.
I also got nice tones miking a 63 Super using the gizmo and the 58 with the ball end off (but didn't compare it to the same without the gizmo).
A really cheap way for those of us on a tight budget to get more from a 58/57. I paid $30 for the gizmo but you could easily solder one up if you have a spare XLR adaptor, or solder a resistor into the plug of a mic cable.
It's apparently no use on condenser mics and didn't help with my other dynamics, which are more modern. Might help with other older design dynamics.
Anyone else tried this?