https://www.cnet.com/news/amazon-echo-speakers-getting-stereo-pairing/

The audio features were in addition to a bevy of audio-focused hardware news. Chief among them was the new Echo Sub (pictured above) which can be integrated into a stereo pair as the ".1" of a 2.1 system, for added bass. The Sub will cost $130. 

In addition to the Sub, other new Echo products unveiled today included a new Echo Dot, and new Echo Plus, the Echo Link, the Echo Link Amp and the Echo Input.