[morzh]

Reversing "speaker leads" if it means the voice coil probably won't help much but could be worth a try.

reversing hum bucking coil will eliminate the possibility of it is adding to the hum instead of bucking it.

The idea of that coil is, it picks up the field created by the ripple in the field coil (ideally it should be DC, but when you use the coil as a filter it will have ripple) and then injects the current of opposite phase and of the same waveform into the voice coil.
If the ends are switched, the phase of that current will be the same with the ripple field and will add to it.

Switching the voice coil leads ideally should not matter.
However if we are to assume the output transformer picks up something (shouldn't - it is a closed magnetic core, but then it does have a gap as it is an SE transformer) from the field coil, switching the voice coil potentially might matter. Though probably way less than the bucking coil.