Step down transformer for vintage tube amplifier
Since the amplifier is mains connected you can use an ordinary step down transformer. An isolated stepdown is nice but not needed. The extra safety introduced helps to prevent electrical hazard in case the input or output transformer of the amplifier fails. 

If all is good then the common return from the amplifier is not connected to the chassis. The chassis itself can therefore be connected to earth in both situations. Without isolation transformer or standard stepdown transformer.

Finally there is no problem in connecting the amplifier to 50 in stead of 60 Hz mains.

Warning. If you are working on mains operated equipment be careful and respect the fact that a mains voltage can kill.