These electrolytic capacitors are designed around specific operating parameters, one of them being the voltage. You can always use a higher voltage capacitor, but IMO it isn't always good practice and beside that, ... why?
When I select a capacitor for a circuit, I select it based on the maximum voltage in use. The capacitor is designed to operate at that voltage, and as we can see with these old beasts heat is what shortens their life, when they're shot they're shot no matter how much higher their rating is than their actual use. If it worked fine for 40 years and the technology has improved, ...
The higher voltages need to have more or better dielectric to prevent breakdown at the higher voltage, and might not have the same reactance at significantly lower voltage.
A 100v / 220uF capacitor is rated at 100v nominal, and is 220uF at that voltage, ... not necessarily at 6.3v. This is its rated operating voltage, not its maximum.
If you take out a 6.3/220 and can get a quality 6.3/220, use that. If you can't, especially in a power application, no harm in increasing slightly.
I would not recommend changing voltage significantly in signal path nor in filters, I was always very careful in designing filters and looked at capacitors very carefully. Changing to a much higher voltage WILL change your filter.
I know that there are many here who routinely replace all electrolytics with at least the next voltage rating up, not sure what they think that they're gaining, and I know that I'm turning this into a debate with my opinion. However, lets see measured data that shows why you should change the rating from the Engineers' original design. Anyone?
Next feel free to ask why many people also somehow feel that the Engineers put smaller value main filter caps in amplifiers than the optimum value for highest output, ... here's a hint: THEY DIDN'T! They put in the best capacitors for the musical performance of their amplifiers.
Now I realize that there are BPC amps out there, the lowest-bidder crap from recent years, but this thread is about the SX-1050.