[morzh]

Well, in fact even my measly Stromber Carlson out of the cabinet did not do it.

Whatever AVC and complexity a radio might have, if a distortion is due to acoustics this is then out of electronics hands short of introducing some acoustic negtive feedback, negative output resistance damping and such...if a speaker vibrates out of the box due to resonances not being damped, then the class of radio is no longer important.
Which is not to say that this is not the case....maybe the sheer speaker in 37-116 is better.


I also found that the 60Hz hum gets reduced quite a bit by installing a .2uF or so cap across the filter choke. I was also hoping this was partly responsible for the distortion as the scope shows well modulated 1kHz by 60Hz. But not much effect other than hum attenuation.
They in fact did introduce this change (a smaller cap for 60Hz) in later revisions, plus changed the choke.