Hi Kaarlo,
There are several a few ways to handle echoic rooms:
1) Avoid corners – never set up in a corner of the room. The 90 degree corner acts as a bass loader or primitive horn. So much of the mud in echoic rooms is caused by bass.
2) Keep the volume low – in any room, people always try to talk over the music. Musicians raise their volume so people can hear the music. Then, people talk louder. It becomes a vicious cycle. The greater the volume in an echoic room, the worse the sound. Resist the urge to overpower a crowd. And when people come and say they can’t hear the music, tell them to put the party in a decent room next time!
3) Try shooting across the room – a lot of the times a small stage is set up for the band. If the room is long and narrow, ask if you can set up roughly midway (not dead center) and shoot across the room. In other words, you want to shoot the shortest distance. Typically, you only want to feed the louder sound to the dance floor. The outer sitting areas should only receive listening levels, if possible.
4) When you're on a raised stage, try to aim your speakers directly at the peoples' heads. The people act as a natural sound absorber. In other words, try
not to shoot at another solid surface. This idea is less important with Bose speakers or other similar designs. Horn loaded speakers are harder to deal with.
I've been designing and building loudspeakers as a hobby for 45 years. A general premise in reproducing good sound is "opposite surfaces should be of opposite materials."
There’s a few hints for you
...Lee