A couple of years ago, my band played in a gynasium and we sounded horrible due to the same problem your having. A few years later and a new band, we got asked to play another gymnasium show. Of course I was very hestitate but the other guys convinced me it was a show and we shouldn't miss it. Plus it was an important show for the other band which asked us to play. Back to the story, well we set up and started playing and to my surprise no feedback and we ended up sounding great that night. I actually got a chance to talk to the guy who was running the sound that night and asked him what he did. Here's what he said the answer would be.

Since you have a PA at your disposal, try micing your amp through the PA system with a uni-directional mic (shure sm57). This will hopefully break the feedback loop. You shouldn't have to turn up your amp much, just enough to were you get a strong signal on your mixer. If feedback occurs before you reach this peak, try throwing a light blanket over the front of your amp and the mic. Ok once that is solved, place the PA speaker in front of the space where you guys stand. Face them about a 35' to 45' degree angle away so that you can't see the front of the speaker from where you stand. This should solve the feedback problem, but you'll still need to follow the advice of Doc and SPL.

If this does not work, it's definitely the room and you'll just have to find a better place. Sorry. Look for something that doesn't have alot of metal surfaces. Concrete is ok, but wood, and dry wall are the best because they all can absorb some of the sound waves. Also if you can carpet the floor that will help with absorbing soundwaves. You need to find a better acoustical environment. Good Luck!

[Edited by noticingthemistake on 10-26-2003 at 11:16 AM]