Currently, what I do is I have a tablet that is paired with my Echo for audio output. The Echo is attached to a wireless (Bluetooth) speaker with a 3.5mm cable that is usually plugged in to power as well. When I want to move the speaker around, I have a 2-inch diameter "puck" Bluetooth 4.0 (can connect to 2 output receivers simultaneously, though the output is not synchronized) that I connect via 3.5mm jack.
This setup allows me to broadcast notifications via 3.5mm cable out or Bluetooth out from my Echo. Also, I have multiple setups like this in different areas of the house.
The tablets have Lannouncer installed, which receives messages from notification Smartapps like BigTalker.
Also, the tablets all have a notification reader app installed. This allows me to bypass configuring a Smartapp to broadcast TTS or media notifications. Instead, just regular notifications are read and rendered TTS by the app, using the renderer I choose and have installed on the device.