An amplifier in the US (you don't say where you are) that claims a particular RMS wattage has to specify the resistance into which that power is delivered. In other markets, trade regulations may be different.
If you're using an "8 ohm" amplifier (a very common specification point) and you need 50VRMS, then without any headroom at all it's a "312.5 watt" amplifier. The amplifier may have a data point at 4 ohms, but that's irrelevant here.
Now, if your load doesn't need to be "grounded", you can use a "stereo" amplifier in a bridged configuration. Two "8 ohm" amplifiers work out of phase against one another to provide double the voltage into 16 ohms (or more.) Each of the amplifiers only needs to provide 25VRMS, which requires a nameplate rating of only "76 watts" per channel (plus headroom).