The introduction of the PX series was a major redesign of the internal structure of the panel to greatly strengthen it, as well as a switch-over to a better stator wire and insulation which addressed a weakness that led to far too frequent panel failures which necessitated a return of the panel to the factory. Since then ‘arcing over’ and the resulting burn holes in the membrane has become a rarity, eliminating a problem that resulted in a reputation for poor reliability. If your non PX panels are still working well, it’s most likely they have not been moved and subjected to the stress of handling. Today’s panels are significantly stronger and more reliable.
The subsequent improvements in sonics have mainly come with the introduction of better toroidal transformers and better capacitors, all included in the “Consumate” backplates and lastly the Bass Focus redesign of the panels which improves the bass response. Older backplates can be updated, pre PX panels can not; they need to be replaced.