Going out on a limb here by getting technical than I should. But I recall reading posts from some our tech members that described the "common" ESL impedance function to be like capacitor. That is high impedance at low frequencies and low impedance (sometimes very low) at high frequencies. Indeed, some of the posts said that only amps that are stable driving such loads need apply.
I think another important question to think about is whether the ESLs were voiced to be driven by a SS or tube amp. As Ralph Karsten (Atmasphere) has explained in his white paper, most SS amps operate under the Voltage Paradigm, i.e., these amps have a very low output impedance and maintain constant output voltage. Under this model, power (i.e., watts) delivered to the speaker will inversely vary with the speaker's impedance, i.e., less watts at high impedances; more watts at low impedances.
By contrast, many tube amp have higher output impedances and operate under the Power Paradigm. In such cases, power (watts) will not vary as much with changes in speaker impedances.
The main point is that as long as the amp is operating within its safe operating range, it's more important to know whether the ESL was voiced to be driven by a SS or tube amp.
Btw, I think Ralph has suggested using ZERO autoformers to simulate higher speaker impedance. The theory is that Zeros can help amps manage the very low speaker impedances presented by some ESLs at high frequencies.
