Forget about "phantom power"! You need to look for leakage.
Take your AC volt meter and put one lead on a ground- like the receptical- and then look for voltage with the other probe. Try the amp case, microphone and shield of the mic cable, any place else you can think of. I would start with the mic then disconnect it from the amp and see if the voltage goes away. That way you will know it's coming from the amp or the cable somehow.
This is best done with a analog VOM. Most digital meters present a high enough impedance that they will say there is voltage when it really is just harmless stray pickup, like the hum you get when touching the input of an amp.
The best way to measure for leakage is with a 50 to 100k load resistor across the test leads. Then any meter will work fine.
I would be very careful here and remove the equipment from service until it's fixed.
-Hal
