I have a 2008 F-250 SD with the factory 8 speaker system and factory amp.

I have great sound through all of the speakers when I have the volume turned up above 9 or 10. As soon as I turn the volume down under 9, the front Left speaker goes out with no sound. If I turn the volume back up it comes back on.

Went to the installer and they checked the wiring and the connections. I also had the problem speaker replaced. No change.

Any ideas on a solution?

I just upgraded Z1 to Z3 and I'm having similar problem. I've noticed considerable volume difference in rear speakers and passenger door speaker and then it returns to normal. No idea why this is happening or what to do about it, just figured I'd post to let you know you're not alone.