I stopped using Amazon Music HD last week when my 90 day free trial was up. They had not enabled exclusive mode at that time. Since my external DAC shows the sample rate it is being sent it was easy to verify the up and down sampling that was taking place in Windows. If you follow the
thread on Amazon Music HD at Steve Hoffman music forum Amazon has still not enabled exclusive mode even with the latest updates. Ask
@Matthew J Poes here, he and Gene wrote the article comparing Tidal and Qobuz.
While I can't say what was what in the past with exclusive mode.
Now I can tell you, when I play a track ( through amazon hd ) it show's the track quality, my device quality and the play back quality that is being streamed back to my device.
When I set up my device and drivers. If say I only check the box for 16/44 then a track being played at below or above that quality will be converted to 16/44. But if I check all the playback boxes it's capable of playing, Then windows mixer is bypassed and all tracks are played back in the quality being streamed from that track. Example a track a 24/96 will playback at 24/96. Also says this app has exclusive control.
Now lets say, (Example ) I leave only the box checked in my device for 16/44 and playback a track on amazon that is 24/192.
The amazon app will show, track quality 24/192, device quality 16/44, playback quality 16/44. But as long as I check all the boxes that my deive is capable of playing. I don't have to change it or adjust for every different track quality. it is automatically done by the app which is doing the stream. So I would say it is bypassing windows mixer like you described. There is NO up-sampling or down-sampling occurring.
So if you are or were experiencing the up-sampling or down-sampling on your computer and or dac. I would say it was due to what version of windows you are running, or your dac. Or as you said this could have been an issue with Amazon hd app in the past. I will also add I had issues with the app when downloaded from Microsoft store apps.
But when I uninstalled the app from microsoft and reinstalled straight from amazon, there are no issues.
I understand exactly what you are saying and I think it's all pretty much true for what was happening in the past.
However on windows 10, version 1909, these issues were fixed in the update on Nov 26th.