I'm assuming you have not tried Spotify Premium and you're comparing a lower version of Spotify to Amazon Music?
Amazon Music is 256kbps, Spotify Normal is 160kbps, Premium 320kps.
Amazon is .mp3, Spotify .ogg - not personally found anyone yet who finds Amazon Music superior to Spotify when it comes to sound quality, as long as you are doing the comparison correctly and have the correct bitrate selected? If you are comparing Spotify Normal or the free version, then yes Amazon Music should should better.
So you might be getting better results at the moment, but bear in mind the above as you are not comparing the two correctly as they have different bitrates. If you are comparing Amazon to Spotify Premium and it sounds better, it's likely you are doing something wrong as Spotify should be sounding superior to Amazon. So before committing to Amazon, make sure you are getting the best from Spotify before making a decision - at the moment you can get a 3 month trial of Spotify Premium for 99p, which might be worth investigating if you already haven't. If you are comparing Premium and have it set up correctly and Amazon Music still sounds better, then just ignore the above
Good luck with your streamer, can't help you there as I have no knowledge.