Hi @Skyhy63, welcome to the Sonos Community!
Just to quickly let you know, I moved your post from another thread to its own topic, as this post didn’t relate to the original thread it was posted in.
I haven’t heard that the Brennan B3 is the only CD player that seamlessly works with Sonos, though it may be one that’s spoken of in high regard. If you were going connect the CD player directly to Sonos, then you’d want the Sonos Line-In Adapter.
I can’t give any recommendations to what CD player you should use but as long as the CD player can connect to a device that is in pairing mode (the Era 300) then you shouldn’t have any issues. We also have a user guide on how to enable and finalize Bluetooth pairing on the Era 300.
If you have any further questions then please feel free to reply here, or list what problems you have seen listed on the Sonos forums so a community member or I can assist.
I hope this information helps!