That is not correct.  Music will play through the 300s.  It’s best to go to Settings (gear icon) -  HT Room - Surround Audio… set Music Playback to Full.  You probably already did this if the SLs were cranking out balanced sound when playing two channel music.

 keep in mind the front firing speaker in the 300 is disabled when it is Bonded as a surround.  Some, not me, are unhappy with that fact.

thanks for responding, do you feel like the atmos on the era 300 makes a significant difference?

 Yes.  I have found several great sounding Atmos albums.  What I notice most is more dynamic range and an increased size of the sound stage.  I listen to a lot of classical orchestral music where dynamic range is most noticeable.  I’ll be listening to a quiet section while my wife is in the room.  She is fine with that until suddenly it’s time for some timpani and loud brass.  At that point I hear the words “ Could you turn that down”!

 I think if you’re a fan of Classical music and Movies the 300s will make a difference.  That difference will be less noticeable if you’re unable to position the 300s some distance behind you on stands at ear height.

 If you do purchase a pair of 300s try them as a stereo pair first.  They sound very good as stand alone speakers.  When you want to later use them as surrounds separate them from their stereo status first.