Tube integrateds and Electrostatic Speakers

You want an amp with as low an output impedance as you can find, to keep the amp from affecting the frequency response of the speaker. One amp designer who takes this subject very seriously is Roger Modjeski of Music Reference. He designs and builds not only tube amps, but also ESL speakers---even one with integral direct-drive tube amps without output transformers! He doesn't offer an integrated, but recommends and will build a passive pre into any of his amps. Better than ANY integrated on the market, save perhaps the more expensive Jadis and EAR-Yoshino models.