[Tim Farrant]

If the output transformer had a grounded centre tap on the primary, would you say the 100uF capacitors are still in series? I think not. Assuming the signals on the output of the opamps are 180 degrees out of phase and equal in level, the centre point of the transformer primary winding has no signal present. Each opamp "sees" half the load impedance presented on the secondary. With a 10k load, that's 5k, which works out to a -3dB point of around 0.3Hz.