[Arran]

I guess I should clarify, it's possible that the filed coil on the original speaker, barring something like finding a dead short across the B+ on the other end, may just have a broken lead at one end, it's worth investigating. I can't remember what style of speaker the model 91 uses but in some of them the pole piece can be pressed out, and the bobbin with the field coil removed and repaired or rewound. This is easier to do on one of the older pie pan shaped speakers that the model 70 and 90 used, I don't know about the later 8'' pincushion shaped ones, on the 12'' console speakers the rear bracket and pole piece is held on by pins that can be pushed out once the cone is removed.
Regards
Arran