Interesting post! I've found the SQ with Spotify (on various devices) is poor. Poorer than I'd expect, simply by virtue of 320 kbps MP3. It's almost as if Spotify are cutting some other corners to reduce their bandwidth. I'm an Amazon Prime member, but don't have the "ultimate" package. TBH, I'm a bit confused about what that is. Just as a Prime member, Amazon seem to give you access to about half of each artist's catalogue. I guess the other half is the carrot at the end of the stick to make you buy the ultimate package?
In any case, there can be a slight gain shift between some of these music players, which makes it difficult to do a proper A/B, because even if you have an SPL meter handy, but the time you've used it the moment has gone and your short term memory has been flushed. On my V30, I prefer Tidal. Amazon's player has the usual Android mixer re-sampling issue, and since all of its catalogue is 44 kHz, it's potentially going to affect everything if you're using high-sensitivity headphones. I'm using the best quality settings, but maybe they get better still when using an ultimate subscription?!? (Since the app is the same, I'd bet any money you'll still be stuck with the usual re-sampling to 48 kHz though.)
I'd love to give up Tidal, because it's expensive and always in the process of imminently shutting down due to something sketchy. But I've yet to find a streaming service that sounds as good.