The funny thing is with most of the comparisons you can tell the difference, despite the frequency responses being similar.
- These are all good monitors with a quite flat response.
- The Yamahas have waaaay more clarity and punch in the mids, high mids, and highs than any other speaker in this price range for a given size; like, so much that they might actually drive you crazy. They are also a bit lacking in the lows, size for size. They definitely bring out the most detail in the mids. Due to this they would make very good monitors but less good for music listening.
- The KRK’s have the most boomy and punchy lows, almost like a regular speaker, which is good for listening but not great for monitoring.
- The JBLs are a good mix of the two, size for size.
- The Kali and Adams are as well, and are pretty great sounding for the price too, though I think the JBLs edge them out.
- You can tell the difference easily between each of them next to the next size up or down in the same line.
So yeah, IMO it’s totally weird that the sub-$200 5" JBL’s sound exactly the same to me as the $350 6.5" Yamahas 
Yeah, choice is clear for me to upgrade. JBL 306P mkII for sure.