Larryi...The design of electronics so as to not rely on precision parts or trims is a well established practice in areas other than audio. In my area of work (missile guidance systems) we were forced to adopt this philosophy, quite apart from cost benefits, because requisite performance was not achievable simply by using more and more exotic parts. There were howls of protest from the circuit designers, but once they put their minds to it things have worked out well.
One obvious example from audio is automatic tube bias adjustment.
As for evidence about gassy tubes, I simply submit that gas leakage through glass is a function of time so old tubes can be expected to have picked up some gas over the decades. Of course a new tube might have gas from the get-go. Some large tubes, notibly CRTs, have a "getter" which captures gas when the tube is operated, and failure to turn on such a tube, or at least the filiament, for a few years can result in catistrophic failure.
