One of the main differences between the Amazon Echo and the Amazon Echo Dot, apart from their sizes, is that the Echo Dot could pair with Bluetooth speakers while the Echo relied on its own in-built speakers. Due to a recent update, however, you can now pair your Echo with any other Bluetooth speaker.
It's unclear exactly when the update was rolled out, however, pairing seems to be a simple process. Put the external speaker into pairing mode, and go the Settings section of the Alexa app. Choose your device, and select the external speaker.
The benefits to this update are that the Echo can now utilise more sophisticated Bluetooth sound systems. The separation of the Echo's microphone from the drivers could also make giving commands while music is playing more accurate.
Though a nice addition for those who already own an Echo, anyone wanting voice-activation functionality without being locked into specific manufacturers (such as Sonos or Tibo) may be better off with an Echo Dot due to its low-cost and inclusion of an in-line jack.