Or ... are you saying the relationship between an amp's output impedance and an ESL's varying impedance plot is simply not a relevant consideration to the ESL's sonic output because of some unique or different electrical characteristic peculiar to ESLs??
As I have pointed out before, with all ESLs, the impedance curve is not a function of a driver in a box. In addition, the impedance curve is also not the efficiency curve.
IOW, the speaker has the same efficiency at all frequencies. Note that I used the word 'efficiency' as opposed to 'sensitivity'. I am restating my second paragraph above slightly, but if I had used 'sensitivity' what would be happening is that the impedance plot would also be a sensitivity plot.
The reason is that the speaker's impedance plot is based on a capacitor.
The result is that an amplifier that operates in the Voltage Paradigm will make too much power at high frequencies due to the low impedance, and usually not enough power at low frequencies for exactly the same reason.
For this reason you will find that a lot of ML users that have solid state amps likely have them fairly close to the wall, which gets some bass reinforcement. Most ESLs, like any panel speaker, should be 5-6 feet from the wall.
I think now you see why the ZERO is so useful when dealing with this speaker. But I will explain: if you use a solid state amplifier one of the really likely results is that while detailed, it will also be noticeably brighter than reality. I've experienced this first hand many times.
Now if you use a Power Paradigm amplifier, as Al has pointed out, the 'output impedance' of the amp will interact with the low impedance of the speaker at high frequencies and you will experience a roll-off. This is why many tube amplifiers (in fact most tube amps) sound a little dark on that speaker if not outright rolled off. Those with negative feedback will of course sound less so (but of course negative feedback causes all amplifiers to sound bright due to trace amounts of odd ordered harmonics being introduced). Now if you just want things to sound natural with no roll off and no brightness, what to do??
That is where the ZERO is handy. It raises the impedance of the load enough that a tube amp can begin to operate within its constant power region. In this way you get high frequency speed and extension, while at the same time getting bass extension and impact.
A long time ago ML made a speaker called the CLS I. Unlike the newer iterations, it had a more traditional ESL impedance plot (meaning it was higher impedance like the older Quads) and it was a breeze to play it with a tube amplifier. As we all know (or should know) transistor amps don't make a lot of power into higher impedances (+16 and +32 ohms), IOW it may well be that such an amp might only express about 1/4 of its 8 ohm power. It is for this reason that a 100-watt tube amp can keep up with a 400-watt transistor amp on Sound Labs BTW.
Anyway, ML realized that if they were to sell speakers to the solid state market, which at the time was about 90% of the amplifier market, they were going to have to do something. The result was that succeeding speakers had much lower impedances, with the 0.5 ohm 20KHz impedance you now see. The ZERO corrects this issue so tubes can be practical on the speaker.
