Have you actually measured 530V? Where and under what conditions?
I can believe that with the standby open you might be getting toward 500V on the first parallel pair of 16uF caps. The DR calls out a 450V rating (BTW caps in parallel have the same voltage rating as a single cap, for caps in series add the voltages). So your 475V one would be an improvement. If these amps have been around for 50 years with 450V why worry about 475V? That said, if you are seeing over 475V I personally would use the 22uF 500V caps as it's well within the tolerance range of the originals.
Don't drop the voltage. It is waste of power and will significantly reduce the power output and change the tone.