This Fostex unit is small, powerful and reliable:
6301 Analogue : Personal Monitors
One thing nobody's mentioning is the need for it to remain really silent when it's not being used...say in a church or a quiet hall.
It needs to be placed close enough to the players to hear it, yet it can't hum, hiss or buzz when the talkback mic is switched off, despite having it's volume cranked to a respectable level.
The last thing anyone wants is a hissy talkback amp/speaker contributing to all the other location noises one has to contend with at a recording session.
This could be less of an issue in a studio...maybe ? In this case it's likely you just have a speaker in the live room, with producer/engineer switching it on in the control room