Electrostatic speakers are not driven well by an amplifier that makes constant voltage with respect to load (i.e. most transistor amplifiers). This is because the impedance variation in ESLs is not due to the resonance of the speaker (where the constant voltage concept works fairly well). ESLs tend to sound better with an amplifier that tends to make constant power with respect to load as they will then exhibt flat frequency response.
Amps with contant voltage outputs will tend to sound bright on most ESLs due to the nonlinear increases in power that the amplifier makes as the impedance of the speaker decreases with frequency. Conversly, the amplifier power drops at lower frequencies where the impedance of the speaker is higher, making for bass-shyness.
In general, your best bet is a large tube amp, which is more likely to exhibit constant power if it does not have excessive negative feedback. Innersound speakers are a bit of a special case, as they have very low impedances that make them a difficult load for a tube amplifier.
This is often solved, as long as the power requirements are not too high, by the use of the ZERO. see zeroimpedance.com
