From the horse’s mouth
Google recently enabled their Assistant and Chromecast platforms to support network-based multiroom audio through compliant audio devices. This has the facility to stream selected online audio sources to the audio devices that work these platforms and permits the use of logical groups as well as party-mode playback of the same source across the multiple devices in that group.
Amazon initially let out an Alexa application-programming interface to permit multiroom audio play across multiple Echo or Alexa-compatible devices. This initially supported logical groups and party-mode playback of the same source across devices in a logical group. But they one-upped Google by adding extra functionality to their Alexa API for multiroom audio including the ability to set up a stereo speaker pair or allow a speaker to be a member of two groups. It is in conjunction with a newer Echo Show device answering the Lenovo Smart Display that is based on Google’s Home Platform.
As well, Amazon had just unveilled new hardware under their brand to take advantage of these new capabilities. One of thse is the Echo Sub subwoofer that can be set up to work alongside a single Echo speaker or a pair of Echo speakers set up to work as a stereo pair for wider stereo separation. It is about adding that bit of extra bass kick to the sound that comes out of those speakers. Then the Echo Dot and Echo Plus speakers have been revised while an Echo Input device was unveilled to put all its audio output via a a connected speaker or sound system.
To connect your favourite hi-fi system to the Amazon Alexa infrastructure, Amazon offered the Echo Link devices which just exist to stream audio content. Both of these connect to the equipment via an analogue RCA line-level connection or an SPDIF digital connection which can be coaxial or optical. They also have both a digital and analogue input connection, perhaps to pass audio devices through the connected sound system, but I am not sure if these devices can stream an audio source in to the Amazon Echo setup that you have established. The Link Amp variant has an integral power amplifier in order to play the music content through your existing passive speakers.
A question that may surface as Amazon rolls the enhanced network multiroom audio functionality across the Alexa platform is whether third-party devices could benefit from these new functions. As well, could Google answer Amazon by offering the extra feature and more for their Home platform especially if they run a range of smart speaker products under their own label? It could simply be the sign of things to come as both Amazon and Google duke it out for the voice-driven home assistant market.