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.
Amplification of classical soloists in concert performances ?
I watched a YouTube video of John Williams performing a movement of Rodrigo's Concierto de Aranjuez with the LSO.
https://www.youtube.com/watch?v=ekznnxaGzNU
I noticed there was a B&W (I think) speaker near the soloist—probably to help the solo guitar match the volume of the orchestra. Is this a common technique for concerto performances?