Amazon has been a bit coy about the mp3 rates they use and have said they vary. I liked the sound quality of their Unlimited tier better than Apple Music (256 AAC). I now use Deezer HiFi, which I like better than Tidal, BTW.
Apple vs Amazon
I'm not fully committed to streaming yet. DAC is low end. Finishing a 3 month free Apple Music trial. In truth, it doesn't sound that bad to me but tonight I started a 3 month Amazon Music trial and just after spending about an hour with it, it seems to sound better than Apple Music running through the exact same system. Fuller. Richer. More precise bass. No A/B testing, just a general impression. It might even be the music selection. I don't know. Is there any actual reason for this to be true? Or is it just me?