For a simple audio monitor, pretty much any speaker will do. Except bluetooth. Won't work. Stay away. You want something wired. On the low end, you can fine a small Hotspot or similar monitor speaker, but that's not the whole solution. As you discovered with the baby monitor, the real question for backstage monitors is how you intend to feed it. Are all of your shows mic'd? Do you have overhead mics on the stage? Do you have a way to take those microphone feeds and route them to your dressing rooms. There are a lot of approaches, but a lot depends on what you have in place to feed the monitor with. You might want to consider a video monitor with a camera output sent via ethernet cable. You can find cameras with infra-red, an a descent onboard microphone for a couple of hundred dollars. Hang a cheap TV and you're done. You do need to confirm that the camera will continue to pass video indefinitely even when the camera is NOT in record mode. Some time out after a fairly short period of time if they are not recording.
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Kirk Longhofer
Theatrical Sound Design, Audio Production, Editing and Engineering
Systems Design and Consultation
www.technopraxis.orgkirk@technoprasix.org------------------------------