[Jean Martin]

I googled some weeks about theory of strings, and I summarize the results here:

If you want to make guitar sound loud and bright in standard tuning, the standard string set simply is the way to go. If you want to think this parabolic shape as a thumb rule, it is ok.

The brightness on a single scale guitar on plain strings is mostly defined by it's pitch, not the tension. The high e is close (perhaps slightly above) the recommended maximum for string pitch on long-scale guitar. So no matter how thin or thick strings you use, the physical rules make it relatively impossible for plain string in lower pitch to compete with the sound of string in higher pitch, if long-scale guitar is tuned in e'. Sooner or later the string is so thick it starts to sound like a bar, and it will be very painful to press.

To test the idea of progression, one should try it with a short-scaled guitar, or tune the guitar in Eb-pitch. This way you could compensate the natural mellowness of b-string by choosing a thicker string instead of the loose one in standard set.

Anyway at the time when first metal stringed Martins were made, they were not tuned in the pitch we now as standard. Probably lower, if I should guess. But who knows, for concerts they were probably tuned as high as possible.

Even though the progressive tension with most manufactured strings at least sounds to create uniform spectrum for all wound strings, the physics here is more complicated. The bi-filament structure is an idea to weaken the string on thick strings to have a brighter sound. Now the choice of core/total weight- ratio should also be considered to estimate the optimal string for each set, the tension being just one parameter here to consider.

On multi-scale guitars though there is more room for tonal adjustments, and in theory can be set to sound brighter or darker, in uniform sound and feel - if that is what you are after.