[Arran]

 On a lower end set like five tuber in the Transitone series i think that leaving the cathode capacitor out may have been a cost saving measure given the market, it's also possible that when this 53-566 was built Philco got a deal on four section caps with the cathode cap included. I have noticed that adding one tends to improve the bass response, sometimes it increases the gain substantially, sometimes not as much, I haven't noticed any more distortion though. I have found that it's more beneficial when the set has a larger diameter speaker, like a 5'' inch or larger, otherwise it gets a little too boomy.
Regards
Arran

P.S Art, if you have lots of 47 uf caps, use them, the pi filtered power supplies in these AC/DC sets are so crude they can use all the filtering they can get, especially on the output filter cap.