04-09-14: UnsoundAs I indicated I have no specific knowledge of the behavior of Martin-Logan speakers. But keep in mind that application of a voltage which is constant as a function of frequency will result in power delivery which progressively increases as impedance decreases. (In saying that I'm oversimplifying a bit by ignoring phase angle effects).
Unless there is some sort of internal eq or the impedance actually compensates for the drivers-crossover/speaker systems inherent deviations from flat frequency response (something that has been done to some degree in crossover designs from other manufactures, but something I've yet to hear attributed to ML designs), I'd hazard a guess that it might be reasonable to extrapolate that the sound output to somewhat mimic the impedance/frequency graphs provided.
As you'll realize, what a speaker basically does is to convert some fraction of the electrical power supplied to it into acoustical power. And as Ralph pointed out earlier, the efficiency of an ESL (power out vs. power in) does not decrease in step with impedance as frequency increases, but instead remains pretty much constant as I understand it. So in the absence of specific indications to the contrary I see no reason to expect flat frequency response in (in terms of voltage) to result in flat frequency response out.
As I see it the usefulness of TJN's measurements is that they provide insight into the DIRECTION in which tonality will be affected as a function of amplifier output impedance. But they provide essentially no insight into what output impedance will be optimal.
Best regards,
-- Al