Paul,
Your amp idea is close to an idea I was toying with a while back. Since direct drive amps are somewhat rare and expensive, I thought why not use a conventional push-pull tube amp and hook up the output transformer backwards. In other words, transform to a higher impedance load instead of a lower one. I never got out the calculator to see if the turns ratios would work out but my gut tells me it would be in the right neighborhood. Voltage rating on the windings would be an important consideration though. I seem to remember Plitron coming out with some toroidal designs intended for use with electrostatic speakers. I would have to research that one some more but one of their wide-band models might make a good choice to mate with your mosfet design.

Good Luck in your audio pursuits!

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Tekdredger