IMO it is a somewhat misleading oversimplification to categorize the ESL57 as being either an easy load or a difficult load.
It is a difficult load in the sense that its impedance descends to low values in the upper treble region, due to the capacitive nature of the impedance. Although even in that region its impedance is not nearly as low as the sub-1 ohm impedances of some other electrostatics, such as many of the Martin-Logan designs.
However, that difficulty is of course mitigated by the fact that most music contains relatively little energy in the top octave, where the impedance is particularly low.
It is an easy load in the sense that damping factor is a non-issue **with respect to bass control,** as Atmasphere indicated, because of its high impedance in the lower parts of the spectrum.
However, it will be a load that is **revealing** of amplifier differences, not only because of its transparency and musical resolution, but because differences in damping factor among different tube amps, and also between tube amps and solid state amps, will have tonal consequences.
Damping factor is inversely proportional to an amplifier's effective output impedance. (I add the word "effective" to indicate that I am referring to output impedance taking into account the effects of whatever feedback may be present). The interaction of the amp's effective output impedance and the speaker's wide variations of impedance over the frequency range will certainly have tonal consequences, especially among different tube amps, and between tube amps and solid state amps. Less so among different solid state amps, since their effective output impedance is near zero in most cases.
In any event, the bottom line with respect to the tube-friendliness or lack thereof of the ESL57 is perhaps indicated by the fact that it was designed before solid state amps existed.
Regards,
-- Al
