“At low excitation levels, the inductance of the output transformer decreases, the phase shift increases and the tops of the square wave tilts. A high-level square wave will appear better that a low-level one at low frequencies. Similarly, high powers at high frequencies will clip any supersonic peaks in the response and improve the appearance of the square wave. The use of a high level of power can make a relatively poor amplifier appear better on square wave tests.” (From: D. Hafler and H.I. Keroes, “Ultra-linear operation of the Williamson amplifier”, Audio Engineering, June 1952, link)
“The feedback is taken from the 16-ohm tap regardless of the speaker connection. This tapped secondary arrangement is extremely convenient when shifting to speakers of different impedance as it does not require a change in the value of the feedback resistor. It is made possible by special transformer design (on which patents are pending) which permits equivalent response on all taps of a tapped secondary winding.” (From: D. Hafler and H.I. Keroes, “Ultra-linear operation of the Williamson amplifier”, Audio Engineering, June 1952, link). This relates to a question if the 16-ohm tap on Hammond 1615 and similar transformers can be used for such feedback properly.