Interesting thread. Thanks all.
Given the ostensibly "Black Arts" involved in conjuring great tones from a repeater, I suspect that even if PP, AP, VC, PD, Roth, etc. knew the admixture of elements to consistently make sweet sounding repeaters, they'd keep it well-guarded (too the extent that these things can remain well-guarded in the sometimes incestuous swiss watch industry).
Perhaps the ultimate factor is a Manufacture's willingness (or unwillingness) to let poorer sounding repeaters onto the marketplace. E.g., perhaps PP is the most consistent merely because it is the most stringent when it comes to quality control and not because it knows some otherwise unknown secret about manufacturing repeaters.
And the truth could also (probably does?) lie somewhere between outright better know-how and stringent quality control.