I have 4 different iPads, including a pro, an iPhone and an ipod touch, all of them show exactly the same behavior: after around 90s of playback, I would notice a very substantial decrease in sound quality, and when clicking on a yellow badge, it would show that playback is done at 'standard'. I have a fast 50mbps internet connection and perfect WiFi coverage at my home, so it's not my internet.
I wonder how you have a different experience, maybe amazon's servers serve different parts of the world differently (I'm in Germany). Downloading however works flawlessly, and at a max. data rate my internet provider is capable of. Only streaming is completely broken...
That must be frustrating as heck. I don't know what to make of it, but your suggestion that the feed in Germany may be different is a plausible explanation. If I experienced any of this downgrading behavior, I would drop Amazon like a hot rock and not try it again until I knew it was fixed.
Someone else mentioned that Ipads function a little differently as a USB streaming server and that they might not be "bit perfect." Using Apple's CCK adapter it will in fact be bit perfect provided your DAC works driverless and its usb input is powered in such a way that it passes the Ipad's power check (you can search Head-Fi and find plenty of threads on this). This is very hardware/firmware dependent, so that is why I listed my gear and the chain of connection very specifically. In my case, when I connected the usb cable directly into the Tanly DDC, it didn't work, but when I used the Regen before it, it functioned perfectly. I suspect that the Regen acted like a powered hub that so many Ipad users say they need to get their DAC working. When I then tried adding an Audioquest jitterbug into the mix, the whole thing was borked again. I then tried bypassing the Tanly and ran the Ipad's CCK feed into the Regen and then into the Gustard's USB input. Nope, nothing....then removed the Regen and much to my surprise, it worked fine again. I then added the Jitterbug back in and it continued to work perfectly. Zut Alors!!
This is maddening, folks, and the only reason I'm sharing this dribble is to impress upon some that not only is usb playback a persnickety matter that is VERY hardware/firmware dependent, but apparently, so too is the performance of Amazon Music HD. If you follow the DAP threads like I have, you will know that the functionality and quality of playback on various Fiio, Ibasso and Hiby DAPs varies widely. (We can reasonably assume that the same holds true for all kinds of playback devices). The Amazon and DAP forums are all over the place on what the real problem is. Some say the DAP software has to be fixed to work with Amazon HD because certain DAPs in their lineup work great with it and others don't. Then there are some DAP manufacturers (Fiio) laying the blame squarely on Amazon's shoulders and still others, like Hiby, that release a firmware upgrade for the R5 that fixes nearly everything.
So, in my (very) particular case, Amazon HD is working quite well in both functionality and sound quality -- enough for me to ditch Tidal and appreciate the savings. Obviously, your mileage may vary.