I'm afraid I am not conversant with TS parameters, so my comments here are general, not specific. AFAIK, the only real way to assess the state of charge of the magnets is to measure the flux density in the voice coil gap directly with a gauss meter. Then the readings, made at several points around the gap, can be compared to the driver spec.

I agree with Earl that the LTV era construction may have paved the way for undersized magnets. The 416 was the smaller magnet woofer to begin with, and these cheapskates had recently given the new, downspeced 804A compression driver a really tiny alnico magnet.