Below is the promised picture, done with two screenshots pasted to each other.
View attachment 38161
The blue line shows the frequency response of a covered SH55. Its coils are matched: 4k and 4.01k. Their inductances are of 1.9H and 1.94H respectively, if memory serves me (the difference being due to the difference of metallic mass in slugs and screws).
The red line translates the response of the uncovered SHPG1 mentioned in my previous post (3.99 K / 1.89H & 4.14 K / 1.98 H for its respective coils).
One can see the extended and smooth high frequencies in the spectrum of the SH55: according to my tests, it's due to the unpotted structure AND to the symetrical coils, as well as to the baseplate without holes below the slugs.
The spectrum of the PG, conversely, has a "knee" due to potting AND to the "unmatched" response of its coils. This response is not due to the DCR of each coil: it's due to the holes below the slugs, which modify the magnetic behaviour of the related coil.
The APH1 (that I've owned as well) has the same kind of "knee" in its response for the same reasons. If necessary, I'll post the screenshot of its spectrum too.
Note that I've not said that anyone was wrong here (especially MJ, of course). Simply, IME, unmatched responses of coils and unmatched DCR's can be different things.
Answer posted for the sake of sharing with all the other Duncan users around the world.