Thanks to all who responded, both on-list and off-list.
Summary:
While READING, the 7980S will always default to whatever it
finds
on the tape currently loaded. If no format can be found (?
bulk-
erased tape?) it will use the setting below.
While WRITING, it will use the desity as set with the CONF 46
setting.
So, to fix my problem I had to set CONF46 to 1600, rather than 6250, and
it worked. :)
Cheers,
Fred