[Arran]

I can't comment on the potential ESR and ripple issues with using electrolytic caps in these sets, but one thing they will not have is the longevity of poly film capacitors no matter how good the electrolytics are. Most of the filtering is handled by the choke and speaker field coils in these sets which removes the AC ripple current very well. The reason why later sets went to electrolytic capacitors was due to cost, they could produce capacitors with a high value that were compact and inexpensive, especially in relation to filter chokes, that brought the same results and allowed the manufacturers to make the sets cheaper and sell them cheaper.
Regards
Arran