Regarding transformers in general, if an increased frequency is applied to a transformer the inductive reactance of the windings is increased which causes a larger volts drop across the windings and a proportionaly smaller drop across the load. So an increase in frequency should not damage the transformer
However a decrease in frequency will decrease the inductive reactance of the windings and lead to an increase of current through the windings. If the decrease in frequency is large enough the increased current flow will damage the transformer
So generally transformers can be used at frequencies higher than normal but not usually below. In your case you are more likely to experience longer term problems with the power amp than the lower power items (pre, source, etc) but it is all dependent on the tolerance and build quality of your units
If in doubt contact the manufacturer(s) as they will be able to tell you if the transformers fitted to their units have enough tolerance to handle both 50 and 60 Hz. Usually in respect to frequency a lot of equipment will operate satisfactorily in the range 45 - 65 Hz without modification
Regarding the voltage ratings, if your units are designed for 110vac then they will either require wiring changes to operate on 220vac (if provision has been made to allow this) or replacement transformers or external step-down transformers (this may not be practical for the power amp side of things)
Hope this helps, regards, Richard
