You need to be very specific about your amplifier make/model.
A lot of units are designed with transformers that will do either, but need to be wired correctly. I've not seen a power amplifier that could switch by a mere switch, but I suppose it's possible. These are usually high current connections so screw terminals are the preferred way to do it.
Otherwise, a step-up transformer will work, but since amps take a lot of power the right one is more expensive.
