[Arran]

They aren't really stressed all that much, most of the tubes used in the 40-150, 180, etc are 150 ma loctals rather then the usual 300 ma octals everyone else was using at the time, the exceptions being the #41 output tubes and the 84 rectifier, the B+ voltage was on the low side too, only about 180 volts on the power output tubes. Now why did they use that metal box? I don't know, but it obviously didn't hurt anything as most of this series of chassis still have their original power transformers, however the output transformers are usually dead on arrival if they haven't already been replaced.
Regards
Arran