Step Up/Down Transformers - Good or Bad?

I have an amplifier that was purchased in Europe and I am using it here in the States. I am having to use a step up transformer in order to connect between the wall and the amplifier. Is there a compromise to the sound with any transformer that is placed between component and wall outlet? Does it all depend on the quality of the transformer? Can one use an after-market power cord and have it yield benefits, even if it is plugged into the transformer?

Appreciate the responses.