If your amplifier draws over 500W then its best that the inverter switches off, else it will burst into flames.
Amplifiers are rated in output power, which should say XX watts RMS any other "power" rating is fiction and sales puff. For a decent amplifier it will be about 50% efficient, so for 50W out it will actually draw 100W of power from the power supply. If the output power is lower which it will be on music the power draw is also much lower, say 20W.
Have you checked the input voltage at the inverter?
Frank