I find that some BT devices have better radios than others. My BT phone earpiece will lose connection after about 15 feet whereas some of my devices might make it to 30+ feet, depending on walls and other electronics that are running. A BT connection is a two-way communication between two devices which communicate with each other via BT radios. Therefore you are limited by the weakest radio.

I'm pretty sure Amazon will see to it that any Amazon device with a single device unlimited music plan will be limited to a single active BT connection while streaming via the Amazon device to third party BT speakers. It would certainly be cool to have multiple BT speakers set up throughout the house and simultaneously stream to all of them, but I seriously doubt Amazon will allow that. But even if they did, the single Amazon device will be controlling the stream and you will have to walk back near it to make any voice commands. I guess you could rig up intercoms near the Amazon device and all the BT speakers. But at some point the cost of buying more BT speakers is likely going to exceed the cost of just upgrading to a multi-device plan.