Difference between SET and push-pull using 300B

High capacitance is not that strange in a speaker cable, as the capacitance contributes to lowering a quality known as characteristic impedance. When the characteristic impedance of a cable matches that of the speaker, you get a little more efficient power transmission from the amp due to less reflections.

The primary issue between SE and PP is the output transformer, although PP often has some additional circuit complexity. But its the transformer that makes the difference.

When running PP, the signal has to go from positive to negative and then the other way. Magnetics being what they are, they resist the change in polarity. This is called hysteresis loss. In a SE amp, all that is happening is that the current through the transformer is changing intensity, but never polarity: there is no hysteresis loss.

Due to hysteresis loss, a little bit of extra energy is required to change the polarity. This energy comes from the low level signal, essentially trading distortion for signal. That is why SETs have such great low level detail. Of course, PP has bandwidth that SETs cannot match.

Take away the output transformer and you take with it the issues of hysteresis loss- and any argument for SET. Then you can have the low level detail and the bandwidth at the same time. To my knowledge though, there was only one 300b true OTL ever built, and it was a prototype. OTLs need different tubes that are lower impedance, and they have their own requirements for speakers, although they can usually drive anything an SET or other low-powered PP amp can.