Brian,1)The question really here is if the distortion components are being increased by more or less than amount that the overall output has been increased in the designated pass band of the alignment. If less, then the TH can be said to be a lower-distortion alignment for a given driver.
2) Maybe it's best to build THs with a passband that gently slopes upwards to compensate response changes at higher SPL levels, rather than opting for a flat passband. Hmm... I'll consider that for POC #4.
1) All the comparisons thus far show TH distortion percentages have higher than BR, so TH can be said to be a higher-distortion alignment for all those drivers. Louder, but higher distortion.
2) BR cabinets loose bottom response (rather than the loosing top response as the TH does) due to "port compression" at high drive levels, so have kind of an inverse "loudness contour" that tends to follow our hearing response.
Subjectively our hearing "flattens out" at high SPL, if the upper bass does not increase linearly, response tends to sound "muddy", too much bass. A passband that gently slopes upwards won't change the ultimate high volume response, that would take dynamic equalization, which Screamersusa employed:
"I also used the dynamic eq in the behringer ultra drive xover, as level went up I rolled the low end off" .
Interestingly, the upper distortion also increases more in the TH, somewhat compensating for the relative reduction in upper response.
Art