Amplification of classical soloists in concert performances ?

I've seen it with theorbo/lute with Baroque music. That style music was meant to be played in small rooms, chambers/camera. Playing those pieces in large concert halls with period instruments creates challenges. Either you replace one to a part with sections of players (difficult with theorbo/lute), or you amplify it.

Occasionally, it is also poor choice by performance group of other instruments. I've seen an alleged baroque performance by Los Angeles Chamber Orchestra with full size piano (yikes!), and the pianist going all out on the piano. Did not even keep the lid on, proverbially or physically. Was atrocious. Killed the wonderful plucking by the lutenist.

They have excellent harpsichord available, but decided to use a not-so-piano instead. Inexcusable. Needless to say, the audience loved it, b/c it was loud.

It is fairy common for classical performances to employ amplification.  When Strathmore opened in North Bethesda (Washington D.C. suburb), I went to a special presentation where they actually showcased their high tech sound system and demonstrated how it is subtly used to improve the sound.  

I’ve seen it with Classical Guitar/Lute.  Not elsewhere