If you run a piece of equipment designed to operate on 100v at 115v (the US standard line voltage) all of the internal voltages will be 15% higher than they were designed for. You very much run the risk of damaging the amp. It will run hotter and especially with an old piece of equipment, components may fail.
As far as noise generated by a step down transformer, that would depend on the quality of the transformer. Terii, the original poster in this thread, said he was using a step down transformer, maybe he can comment. In his case he would be stepping down from 240v to 100 volts, a much bigger drop.
When shopping for a transformer, be sure to get one with enough capacity for the maximum draw of the amp. That value will usually be on an ID plate on the back of the amp.
- Pete