I am wondering though how can the ST16 unit can become un-calibrated?
As Steve mentioned, there's a lot going on in there... And, unlike a phone, most of the parts you interact with on the ST16 are physical buttons, pots and switches. They will — guaranteed — drift and/or fail over time no mater how careful you are. As good as the overall build-quality on the equipment may be, the components are sourced from god-knows where; and they ain't made like they useta was.