It's not feedback, it's latency caused by audio travelling at the speed of sound, about 1 foot / millisecond.
I can't do it w/o cans on if the speakers are more than about 20 feet away. 250 feet is a quarter of second delay from the time you speak until you hear it in a stadium. That doesn't even factor in the reverb time. The RT60 in there has got to be in double digits.
People get used to the sidetone in telephone communications where there is essentially no discernible delay on their voice in the earpiece.