Finally got the Mid Arm working!
I guess the Bernina 820QE is thread snobby, because it balked at everything except Superior. New bobbin, rethread and voila, I'm off to quilt-land. Lots still to learn - I didn't leave enough leader free on the take-up arm so couldn't get the stitch I wanted on the border, but those are lessons learned.
Got my first real quilt loaded on the frame and ready to roll - now I'm too tired to do anything tonight LOL.