Mixit's link tells me what I have always thought to be the case, "modern" magnets, be they ferrite or Alnico, do not lose energy over time unless exposed to a large current pulse or (not mentioned?) mechanical shock and that last is unlikely in a cabbed driver since the cab would be destroyed before the "G" was high enough to de-mag the unit even slightly.
I have read an article (by Celestion, will try to find it) re the "ferrite/Alnico" sound debate and there IS it seems a difference but it is small and comes down the fact that Alnico is an electrical conductor and ferrite is not.
I doubt any VALVE amp you could lift onto the van could put enough current thru a 16 Ohm coil to cause magnetic compression and in any case that would be insignificant compared to the THERMAL compression that must accompany such a high drive.
Bottom line: I doubt ANY speakers ever need their magnets "re-charging"!
Dave.