[Arran]

 Actually there are many advantages to owning a set equipped with a 25 cycle compatible power supply, as most have noticed the power transformer is larger, because the core is larger in order to avoid saturation at the lower line frequency. However in addition to the power transformer they would add electrolytic caps of a larger value and often add extra filter chokes or increase the size of the field coil or of the filter chokes to eliminate the 25 cycle ripple. So if you run a 25 cycle set on 60 cycles the transformer will run cooler, can tolerate more abuse, and there will be next to no hum.
Regards
Arran