Marshall wrote
Our
system is: AKG C 3000 B mike, sometimes a AKG C 1000 on the acoustic
bass at very low gain, Mackie 1202 board, JBL EON G2 Powered speakers.

Not the best microphones in the world you have there, those JBL EON's should
work fine. If your Mackie 1202 board has a sweep for the midrange try sweeping
to about 2300Htz and take out as much as you need. There are lot's of other bad
frequency's but that is one that will feedback quite a bit.