Going by pictures of your amp I could find, your IEC receptacle is just like on my Luxman integrated. A normal 15 amp 3 conductor IEC plug will fit, but since there is no ground prong in the receptacle, nothing to connect and no modifications required. As erik_squires stated your amp is double insulated and does not need the chassis ground.
Using a 3 prong plug will preserve the Neutral vs Load polarity since it can only be plugged in one way. Some two prong cords are not polarized (with one wider prong), and can be plugged in either way. That might be part of the reason for the caution regarding cord substitution.