[Arran]

 It is always my first order of business with any of these pre war sets to replace all the paper caps, filter caps (paper or electrolytic), and any resistors that have drifted more then 20% off value. Before I do this I usually test the tubes first, shorted, dead, or weak tubes will usually be smoked out on any decent tube tester. However I don't usually going into swapping out tubes that otherwise test alright until I electrically overhaul a set and am still experiencing problems. The Philco pie pan style speakers all have a common handicap in design, they used a solid center mounted spider that really reduces the frequency response and fidelity compared to something like a a Rola or an Atwater Kent speaker from the same era. The plate or power detectors do introduce some distortion, but even with diode detector these sets would still sound crappy compared to a Majestic or RCA from the same year, also the distortion from the detectors was probably less of an issue at the time since the average radio station put out between 10 and 15 KW of power compared to about 50 KW today.
Regards
Arran