No problem. If your amp only has a two prong male connection, then it will be double-insulated and will not use the chassis as the common ground; it's been designed that way, so probably best not to start trying to earth it yourself.
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Yes, there is no problem electrically. But I'm not sure that a replacement cord with a 3-prong female socket will mechanically mate with your amplifier. The statement about a C18 inlet accepting a C13 line socket I believe refers to the fact that a 3-prong wall outlet will accept a 2-prong plug.
A follow-up to my previous post. I took a look at what was probably the same Wikipedia writeup that you referred to. If your amp inlet looks like this, then you should be ok:
My concern had been that there would not be sufficient clearance around the inlet to insert the part of the cord socket that has the third pin.