Regarding the measurements cited in George's two posts just above, I would emphasize that the frequency responses and rolloffs that are described apply to electrical signals at the amplifier output/speaker input, not to the acoustic output of the speaker. As I said earlier:
Regards,
-- Al
01-14-15: AlmargThe successful pairing of tube amps with electrostatics by many experienced audiophiles, as can be seen in system descriptions and posts here and elsewhere, confirms the importance of distinguishing between flat frequency response into the speaker, and flat frequency response out of the speaker. They don't necessarily go hand in hand, particularly in the case of electrostatics. IMO.
... while it is true that when working into an electrostatic such as the Ethos the use of a tube amp, especially one which uses minimal or no global feedback, is likely to result in voltage vs. frequency characteristics at the input terminals of the speaker that are less flat than would result from the use of most solid state amps, it by no means necessarily follows that the frequency response characteristics of the acoustic output of the speaker will be flatter in the latter case. As can be seen in the comments by Ralph (Atmasphere).
Regards,
-- Al