Great news! As regards non-linearities in the volume control, when I recalibrated with the volume at 0, my speaker trims remained the same relative to each other as when I had previously calibrated at -15, so at least on my current 950, the volume control seems to remain accurate between channels at different levels.

I'll let you know how it works out. I'd kind of like to send mine in for the upgrade, as it is in mint condition, cosmetically and otherwise, but then I'd lose the ability to do a direct A-B comparison, and that I REALLY want to do.