Last September the ceilidh band I play with (the Greenbank Buskers) were doing a gig in a hall which was filled with lots of noisy dancers (etc!). We have usually managed fine in the past but found that the fiddles were having difficulty hearing the accordions, and vice versa. We had amplification but only for the hall.
Having done other gigs where we had (gasp!) a sound tech and even (gasp again!) floor/stage monitor speakers, I knew there was a solution to our problem – but commercially at cost – and we are just a band that plays for charity. However, I remembered that I had an ancient guitar amp I made with a couple of really nice speakers in it …