How big is the amp? In my experience, you will need a step up with at least 50% higher power rating than the amp's max power consumption to avoid degradation. In other words, if you amp consumes 500W at max output, you will need a 750W or higher step up.
Also, make sure you buy high quality. A lower quality transformer, even at the same power rating, will generate more heat. More heat means more power loss which translates into more voltage drop as power demand increases.
Also, make sure you buy high quality. A lower quality transformer, even at the same power rating, will generate more heat. More heat means more power loss which translates into more voltage drop as power demand increases.