Special thanks to Calvin at the DIY Audio Forum for sharing his experience using inexpensive 230v/6v power toroids in ESL's. They sound fantastic at about one-tenth the price of the specialized ESL toroids from Plitron or Aplimo.
Finding suitable tranformers at a decent price is usually a problem in the US, where 230 volt toroids are generally not available.
These 230V/6V European toroids are my first choice and the price is unbeatable too.
I would ALWAYS choose a transformer with a single 230 volt winding over one having two 115 volt windings because I think they are more robust. That said, these 2x115V/2x6V toroids are available fast from Mouser in the US and they sound very good:
Triad Magnetics VTP12-4170
Yet another transformer option would be a quad setup of these Antek AN206's, wired as shown in the schematic below. Although I have not personally tested them, I see no reason why these transformers would not work fine and they are readily available in the US.
Transformers & DC Bias Supply:
The schematics below are color coded for the transformers shown. If using different brand transformers, follow the phasing dots and make sure the (+) phase output goes to the rear stator. This will ensure the woofer will be in phase with the panel. Even then, you should verify the woofer/panel phasing as described here.
Schematic with preferred VTX transformers:
Schematic with Triad transformers:
Bias supply wired for 230 volt input: