FWIW I've had varying luck on Amazon; I think it's based on the publisher of the content. Some of Amazon's own stuff works in 1080 on my machine while other movies are much lower quality
Changing the UA to Firefox windows works too. I always get the "best" stream. Also prime music does not work on Chrome/Firefox on Linux solely because they've blacklisted the UA. I keep yelling at them every 2-3 months