Opinions on the OEP Z1604

[abbey road d enfer]

That is true, I was using a 50 ohm source impedance generator, but not quite zero, I could make a buffer with near zero impedance and try out the results but I dont think they will vary greatly, THD will still be very high.

That's correct; the winding's DCR of 100 ohms is a limiting factor there.

I tried the zero-field method and it works great, managed to keep the distortion low even at high input levels, however the opamp needs some taming, mine started to oscillate, I cured it with a simple capacitor in the feedback loop.

It's quite common in ZF drivers, to revert to normal voltage drive above a few hundred Hz.