You may already know that they can only play Amazon Music, Spotify, Tunein (plus a couple of others) in stereo. You can’t use them as a stereo pair of Bluetooth speakers for your laptop for example (much to my disappointment) and any of Alexa’s skills only play audio through the echo you spoke to (usually the closest) and this includes the news, games, ambient sounds (even ambient sound skills that boast of being in stereo).
That said, Our skills do not play in stereo on Paired Stereo Speakers. There is nothing that can be done about this until Amazon fixes the Bug in their system. Your stream will sound just as great on any single Alexa Device as it does on your streaming website or mobile phone.
Very few sets of Paired Stereo Speakers with a Sub-Woofer have even been sold. Therefore, at this time, I wouldn't worry about it.