the KN6500 doesnt have the "monitor settings" option in the sound menu - i just checked.

headphone jack could be the cause? headphones do work through the headphone jack so maybe it could just need replacing... a headphone jack im sure is just universal not pty technics part... hopefully smile
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Yamaha PSR SX900 / Roland G70 / Roland BK9 / Korg PA700 / Roland GW-8L / Roland Fantom O6