[Arran]

Large value capacitors were expensive, and even after electrolytics came about the values seem to have been limited. It's not unusual to see a mid 1930s radio with a 4 uf  input cap, and an 8 or 10 uf on the output. I've sometimes wondered if the small values they had available were the reason they would have another filter cap added in the B+ to the front end of the radio? The reason they continued using field coil speakers may have been the lack of decent permanent magnets, or in the case of the pre war sets, a lack of materials to manufacture aluminum-nickel-cobalt P.M magnets, or just that it was the convention so long that they had a large inventory of field coil speakers available. I recently picked up a post war Stromberg and it has a P.M dynamic, with a pi filter in the power supply with a power resistor in place of a choke (not really a good idea given that the set had a pair of 6L6Gs in push pull) but have seen other makes and models with the field coil speakers. I think that Stromberg manufactured their own speakers, at least for their upper end models, so they didn't have to worry about parts shortages, whereas Electrohome did not make their own so they used field coil speakers made elsewhere. 
Regards
Arran