Possible to use Sonos Speaker with hifi system?

If you haven’t already purchased the A8 yet it is possible if you go with the WIIM products. Get a WIIM ultra for the main system and a WIIM mini for the Sonos. 

You could use the fixed RCA outs on the preamp and plug them into a Sonos Connect inputs.  The Connect would sit with your main system and the Connect would, well, connect to the Play5 through their own network and you would just control the Play5 through your Sonos app.  Pretty easy.