I agree. It’s very much like phono cartridges in my opinion. You have some pin compatibility and shared function. The branding issue is also true. Western Electric has some correlation to history, but even then the modern ALD carbonized nickel makes it a different product from before.
The other problem is tube lot to lot variability (my WE300B don’t seem all that well matched despite paying the extra money for matched pairs so it must mean that unmatched tubes are even more different!). I have been lucky with NOS US production tubes when using my tube tester, but even then, measurements on NOS tubes are only going to be accurate for the specimens in hand and cannot be generalized.
I definitely am looking forward to your write up of the 91E. It would be great to get your insights on the lack of standardization. At least back in the day, you had KT88 and 6550 as different descriptions. Today, you can imagine a companies changing glass shape and having a different label printed on the glass!
Out of curiosity, the Canary Audio monoblocks boast extremely low distortion. It makes me wonder if those amps show any similar dynamic frequency response changes or if they show more solid state like behavior. I don’t see any deep dives of those amps.