Interesting to note that on The Madcap Laughs a lot of Syd Barrett's acoustic guitar is actually and unplugged Telecaster played into a microphone.

I saw The Acoustic Strawbs a few years back and my ears were aching for days. I especially liked their use of e-bows which can be played on acoustic guitars.

I was once (or twice or even thrice) criticised by a self-styled purist (is there any other sort?) for my use of an electric shruti box in an otherwise acoustic setting. I must add I eschewed the use of the house PA that night...

Saw Hesperion XXI in a packed largish venue at the York Early Music Festival last summer - no PA.

A good device for singers is the Boss Vocal Performer VE-20 on which you can add harmonies, loops & delays etc. Here's a link to Long Lankin recorded at a recent gig we did at The Musician in Leicester. The sound-man at this gig was especially thorough, which was a delight. My violin is going through a Line-6 delay modeller & a harmoniser peddle.

https://soundcloud.com/rapunzel-and-sedayne/long-lankin