[Mondial]

That's an interesting explanation and it makes sense, but I have seen original speakers with push pull output transformers also equipped with bucking coils. 

The way I always understood it, the bucking coil was used to cancel the hum caused by using the field coil as the first filter choke. In this case, the large ripple current component modulates the magnetic field and causes a hum voltage to be induced in the voice coil.  By connecting the bucking coil in series and out of phase with the voice coil, the induced voltages cancel and the hum is reduced or eliminated. 

In a lot of radios I have worked on, if there is a separate filter choke after the rectifier and then the field coil as a second choke, the bucking coil is not used. In this instance, there is little ripple current in the field coil so the bucking coil is not necessary.

In any case, it would be a good idea to try it both ways, with and without the coil to see which resulted in a lower hum level