I had a few legacy Bose SoundLinks lying about. Thought this might make them useful but be aware that the Bose SoundLinks need the AUX button pressed before any input is accepted from the 3.5mm input. This made the Echo Input less than ideal for this setup.
Maybe just early days but when compared with Echo or Echo Spot, the microphone is not as sensitive. Probably uses the same inferior microphones found on the Echo Dot.