It would be a lot easier for me to diagnose the problem if I lived relatively close and could see the setup first hand. I suspect there is an impedience difference or mismatch that is causing the problem, but I really don't know for sure. If there is someone in your area, maybe Tony Madelone, that could hook up their keyboard to the Z-5500's same input it would provide a definitive answer. Kinda difficult for me to figure this one out from nearly 400 miles away. Wish I could do more.
Good Luck,
Gary
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Travlin' Easy
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)