I have a SM12A that I use as my primary mic and on-stage monitor when I perform in my band.
I would like to add some kind of volume control that would allow me to adjust the volume at will on the ear piece? I currently plug the 1/4" plug into one of the output plugs in our stage monitor.
See the SM12A User Guide. It shows a schematic of a simple volume control that can be built with Radio Shack parts.