IMO, one of the key benefits of Amazon Music Unlimited is to be able to stream tracks in Dolby Atmos to experience the spatial format as it was originally mixed (as opposed to the spatial effects an AVR may artificially add). When you turn Spatial Audio off in the app, then you are downgrading the track to stereo PCM. Sure, as
@Martin_320 wrote, Atmos may currently be streamed in lossy format, so turning it off may enhance playback quality. But what really should happen is Amazon fixing the still low volume of Atmos playback (about 16 dB lower than PCM of the same track). This is likely caused by Loudness Normalization hardcoded to on for Atmos tracks, regardless of the overall app setting.
Also, in the latest version of the app (updated a few weeks ago), there is still a bug that the setting for Loudness Normalization is misrepresented. After you disable normalization and reboot the device, normalization is presented as enabled, but behind the scenes it is off (as configured).