[Antipodal]

Hi everyone,Yes N.Z. is on the "Circum - Pacific Belt of Fire " so earthquakes here are not uncommon.We have learned to live with them as best we can  and also to engineer our way towards mitigating them using N.Z. developed  lead and rubber base isolation bearings to allow us to build some taller buildings. 
Regarding electro dynamic speakers you are  on the money in radios powered from 117 or 240 volts (our N.Z. standard) where,as you say, the field coil serves to help smooth out B+ ripples and commonly appears wired with the first cap to the rectifier cathode.
However in most car radios that use e.m. speakers the field is wired ,as you suggest Arran and Mr Fixr55 ,in  parallel with  the filaments and so offers no smoothing function. In Philco sets of the Thirties and Forties from my reading of the circuit diagrams (Philco S4627 Studebaker,Packard P 4635) these field coils are only 4 ohms.Fed by 6 volts they draw 1.5 amps.
With the increasing demands placed on battery and generator as cars in the early fifties were fitted  with  power everything including air con it is small wonder that e.m. speakers faded away.
But they do sound nice when in good condition.