Hum eliminators only work when placed between the offending pieces of equipment. And since powered speakers tend to be plugged in on other outlets/circuit breakers and or 120v legs than your board and other equipment is, my money would be on a problem between a speaker(s) and board. Flourescent lights and light dimmers and fan speed controls being on the same circuits can cause problems as well.
You can read more about it here
http://www.epanorama.net/documents/g...pa_advice.html
I'm not trying to lecture here but in the interest of having all the facts:
Just something to ponder when liberaly using 2 prong adaptors. Without the adaptor if a piece of equipment develops a ground fault it will trip the circuit breaker. With the adaptor anything metal connected to it becomes energized and just sits there waiting for a path to ground to come along. That path may be you or one of your singers.
Worst case scenario: Some old guy with a weak heart or a pacemaker get's a nasty shock and dies, you are now guilty of negligent homicide because you defeated the safety features that would have protected him.
Ultimately it's to each his own.
Sam