I remember Flowers And Frolics Playing a Late Night extra at Sidmouth Festival (and other gigs) unamplified in the mid-1970s, but they succumbed to electronics eventually, allegedly because the melodeon players got fed up with breaking reeds!

Marilyn is quite right that it all depends on how quiet people are prepared to be. I sometime encounter discussions about requirements for PA vs. seating capacity - but it's meaningless. The Royal Albert Hall has a capacity of over 5000 but you can hear a solo singer or violin unamplified - the reason is that whole audience shuts up and listens - and yet in a pub you might need amplification for an audience of 20, or at a festival venue where there's a bar or other source or noise nearby or even in the room.

"feeling like your eardrums have met in the middle of your head"
Well, we use a PA with our band, but the criterion for setting it up is making sure we're audible at the back of the room, not turning it up as loud as possible. It can be hard if you have a drum kit on stage - we don't, and as a result the sound levels on stage can be quite moderate and my ears never hurt.

We did take extra speakers to a gig in the main hall at Cecil Sharp house because it's a big room, but it was a "social dance" type event and we found they wanted it really quiet, even lower levels than we usually play, and the speakers were quite unnecessary.

Anahata