A 25 cycle set will run fine on 60 cycles, better actually, I'm not sure where the idea came from that they would not. Half of the sets I own have 25 cycle transformers in them, they typically have a larger iron core to reduce saturation, and they also had larger value filter caps and chokes to remove the 50 cycle ripple as opposed to the 120 cycle ripple in a 60 cycle set.
Regards
Arran