Pistonic movement sounds like motorboating, it's possible that any instability is very marginal and only happens with music applied. However it does seem that what I'll call the wideband units were receiving a bass signal they don't normally see. Am I correct in thinking the widebands have no crossover or at least no low freq filter? That should be best for sonics. Possibly bass that you were playing (loud, yes?) was showing up as wideband cone movement but the driver either cannot audibly produce bass at low freq or at only a very low level.

