Bart, first and foremost, I would do a master system reset, which should be done by going through the Function, System Restore, and then be sure to place a checkmark in every category box on the left side of the page before performing the reset. This may solve the problem, but for some reason I don't believe it's a software issue.
Next, change the speaker wires. Even if they are brand new. Sometimes this solves more problems than you can imagine.
Also, I would give Frank a call and make him aware of the problem. Frank has an awesome service record and will help you in any way he can.
Good luck,
Gary
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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.)