Wait a minute…
….the Core and the A9 can both be part of a B&O multiroom setup - of course the Core also need the speakers.
If one wants to stream from external to these devices, it is possible via Airplay 2 or Chromecast (depending on ones priorities) - e.g. for Tidal and Apple Music. The B&O devices also support Tidal Connect with the recent update.
You can use B&O Radio and the built-in sources on both and let the second join the first, if you have already started playback.
The big question here is how to make the cable management, if you prefer the physical connection.
Since there is no PL-out in the 19 and only one PL-out in the Core, you will have to use a splitter (and do the bas management via the controls on the BL19). All doable, but needs to be well planned.
Sure, the easier way would be to go wireless/WiSA.
However you will then need a Transmitter 1
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And - as mentioned - WiSA is not everywhere the optimal solution.
I know of no B&O device - apart from the Beovisions/BS Theatre - that has WiSA built-in……stay away from the BS Moment, please.
It should be possible to use the Ever Solo as a player for the BL17/19-Transmitter combo - toslink out and toslink in.
But you will not be able to stream from the Solo to the A9.
You can stream from extern (via a mobile device) using Airplay2 or Tidal Connect to either one or to both.
I am not sure how the volume control on the Ever Solo using toslink out will work???
Using the BS Core it is much easier, when it comes to volume control….just use the B&O app and/or the Beoremote One.
Hope this helps a bit.
MM