You'll want some power for speakers like that! Usually you can add 6dB to panel speakers because much of the sound is passing by a mic only 1 meter away and isn't picked up by it. But at 96dB in most rooms you'll want a clean 50 -60 Watts if you want to get the most out of the amplifier!
An alternate that is about 6dB more efficient is the Popori XR1 electrostatic. It has the efficiency that you would need 96dB 1 meter, so add 6 dB...) if you plan to use an SET that makes 30-50 Watts (at full power). But I think you'll find that such amplifiers have troubles with bandwidth, which can introduce phase shift in the region of vocals. Phase shift over a band of frequencies is interpreted by the ear as a tonality. But with a speaker that easy to drive you could have an SET of greater bandwidth and therefore more musical by simply getting one of lower power.
That points to a design flaw (likely power supply related) just so you know. The rectifier should have very little effect on the sound. What's important is the power supply be stable and low noise without any artifact from the transformer and rectifiers ('swept resonance', resulting in noise for those with a more technical mind).
Output transformers are commonly used to convert from single-ended to balanced; any transformer can do that task. In any balanced line preamp where the circuit is internally balanced as well (please note the distinction) ground is referenced. It would be very difficult to design a circuit wherein this was not the case. Ampex made the 351 tape electronics which were single-ended internally through most of the design except right at the output. The record side was zero feedback.
A good line transformer at the output of a preamp is a very good way to solve the long cable problem. You'd need one at both ends if the amplifier is to have a single-ended input.
