[Arran]

Pete;
  As you may know by know your set was built by Dominion Electrohome in Kitchener (formerly Berlin up until 1915) Ontario. Unlike many manufacturers Electrohome owned their own furniture company called DEILCRAFT, which meant that they could pretty much build whatever they, or in this case the T. Eaton Company, wanted for radio cabinets.
   Part of the reason for the good sound quality may be the fact that they used a Rola made speaker in your set, Rola also made speakers for Zenith, and a number of other companies, in the U.S and Canada, whom unlike Philco and RCA, did not manufacture their own speakers. It could also be due to the cabinet design, the output transformer, and the audio output circuitry they chose.
  I have no explanation as to why Electrohome chose to put a metal shield over the 6V6G power output tube, only that in that model year (1940-41) they manufactured a number of radios with that same feature, including Phonola, Serenader, and Minerva branded models. It really doesn't make much sense from an electrical point of view, unless they thought that it would help the tube warm up faster. By the way, why does the tube next to the 6V6 need an adapter?
Regards
Arran