My 990 is upgraded at last. Thanks to all for encouragement and advice, and especially to Steve who talked me through it on the phone. Removing and replacing the power cable after the Device Restart message instead of just clicking the master power switch seemed to make the difference, thought I thought that had been one of the things I'd tried before. Anyway it worked this time.

Was anybody else surprised at having to name all the audio inputs from scratch (i.e., blanks on the display and menu) when you went to reconfigure after upgrading?