A few comments from an AMD fanboy:
Your review suite seems a bit slanted performance wise. Just looking at it I got the impression the 1660 is WAY faster than the 590. However for example at PC gamer's review,
https://www.pcgamer.com/nvidia-geforce-gtx-1660-review/ over their 13 game suite at 1080P, they have the 590 a smidge faster than the 1660 in 97th percentile minimum frame rate (85.0 for the 590 vs 81.2 for the 1660) which shocked me after reading your review. They have it at 110.1 for the 1660 and 109.3 for the 590 for average FPS, so a virtual tie, but typically the minimum lows are more important. Anandtech's review just glancing over also seems to place the 590 almost on par with the 1660. Techpowerup's review places the 590 at 92% as fast as the 1660, but Wizzard's reviews do typically underrate AMD performance as well for some reason. I also checked Toms hardware review and again, they have the 590 and 1660 basically tied in contrast to your review. Specifically the 590 was faster in 6/12 games and the 1660 was faster in the other 6/12 (just glancing so I didn't run percentages but in most games they actually appeared very close no matter who won). I also liked Tom's review in that they seem to have a very forward looking/modern benchmark suite, including such games as Destiny 2, Far Cry 5, and Metro Exodus.
Finally the 580, 570 (some models), and 590 all have 8GB RAM. IMO 6GB is cutting it a bit close. I dont think it's a big deal at this performance level, but it is something to consider looking to the future and even the present. I think most sites decision to test this at only 1080P (I bet they tested the 590 at 1440P) was partly to protect Nvidia from running out of RAM at higher resolutions.