Headphones will give you a false sense of sound versus an open air speaker system......plus you have to tink of what that piano will sound like IN A MIX also...Your gonna have to go to the store and bring your KB & play thru different systems to get your answer at performance volumes,...do not listen to anyone here..
only your own ears will give you what you need
sound-wise..hopefully your talking Line Array system in stereo also. Good Luck.