I know you mentioned that you have a good ground, but it sounds a lot like the venues' grounds are not actually connected to anything and are floating. The capacitance in the windings of the speaker's power transformer will cause its chassis to float up to about half the line voltage, in your case, 100V. There shouldn't be very much current available, but the high static voltage might be able to destroy output circuits connected to it.
GTD
I think you are on the right track.
Usually when you see around 120 volts appearing in strange situations like this it’s caused by a missing earth.
For example where the earth wire is disconnected at the start of a cable it can act as a centre tapped capacitor – think in terms of the Active being on plate of a capacitor, the Neutral another, and the Earth as a middle plate. You can try it with an extension cord – disconnect the earth at the plug end and then measure the voltages at the socket end.
There will be (about) 120V between the earth wire and active and the earth wire and neutral.
If your speaker has the metal parts/ input connector earthed, the supply has 3 wires and the earth is not connect at the start of the cable this can happen. When you plug it into your mixer (which is earthed) you will find 120V with enough current to give you a noticeable shock.
I would be looking for something like this.