I spy a big error here - amplifiers are NOT needed in concert halls if you have the right instrument. I had the honour to listen to the great Segovia in a concert hall with more than 1000 people, and he played unplugged.
In pubs you also don't need amplifiers; you can play louder, and people will listen to good music, especially when and if they came for this occasion.
I'm a strong advocate for unplugged music - as I often tell the colleagues from the electric branch: When the fuse blows, you have to stop in the dark. We shall continue playing by heart.

IanC has said all that is necessary about acoustic = concerned with hearing. As the opposite of amplified I prefer natural.