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.
I also dont like how reviews focus on 1080P for some reason. I use my 8GB 570 on my 1440P monitor, and it works fine. I dont care about 60FPS. Most even the latest and greatest games run easily on my 570 at 1440P at well well above 30 FPS, at high or highest type settings. Honestly if I was to tweak I'm sure I could run at 60 most games at medium to high, if I cared about that. The 1660 is way more powerful than my 570! So 1440P should definitely be tested.
And if you say 30 FPS is unacceptable, how come the highest rated games are Sony console exclusives that only run at barely 30 FPS? For example, God of War on Playstation 4 runs at 30 FPS with dips under 30 FPS and received over a 90 metacritic. Uncharted series, Infamous, Horizon Zero Dawn, all run at only 30 FPS.
Finally for prices, you list the 590 at $260 and the 580 at $200. These aren't off by a ton but just looking, the lowest 580's on newegg are $190, and 590's are $240 (to be precise, there are currently 4 models of 590 available on newegg, 2 of them are 240). This would skew the value charts somewhat. You seem to consistently overprice the AMD cards. On the other hand, Nvidia prices are rock bottom, you will never find a 1660 for less than $220 which is what you have it listed as.
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.
Of course that said the heat, noise and power usage of the 590 makes it not really in the debate vs the 1660 IMO even if they were the same price or the 590 was $20 cheaper. I may be a fanboy but I'm not irrational. However the 580 and 570, especially the latter, still have pretty strong price/performance niches IMO. Nvidia didn't upset the apple cart too much, they never do, gotta protect those margins.
