Toroidal transformer voltage breaks down
The OPA548 is supplying a sine wave at 50 Hz and 16 Vpp.

This won't work. You have not factored in the magnetization requirements when driving the secondary.
Ignoring the primary (high voltage output winding), the secondary acts like an inductor and the inductance will be too low to adequately magnetize the core of the transformer without high reactive currents that the OPA548 cannot be expected to supply. The primary winding might have a magnetization inductance of 10 henries and, it has this to avoid massive current draw from the 400 volt AC supply when connected conventionally.
The transformer ratio is 400:12 hence, the inductance of the secondary will be (400:12)² times smaller. So 10 H might reduce to 9 mH. That's the effective magnetization inductance of the secondary.
At 50 Hz, that has an impedance of 2.82 ohms. Basically the OPA548 has to drive that impedance adequately at 12 volts RMS. That's a reactive current of over 4 amps RMS and beyond what the OPA548 can deliver.
You should also consider that the OPA548 output voltage is limited compared to its power rails. If your supply (for instance) is 20 volts then you cannot deliver more than about 13 volts p-p at the output under load conditions.