Bill (Grannyring), regarding your comments about humidity being a significant variable, here is some remarkable confirmation, from this current thread:
-- Al
06-17-14: GeorgelofiBest regards,
... this [humidity] is greatly overlooked. Take any hybrid ESL speaker for instance, they are tuned for a certain sound by the factory. The esl panel on a day when the humidity is 70%-100% can loose as much as -3db efficiency, making the balance sound bass heavy as the dynamic bass driver has not lost this -3db.
I have measured it on my new panels, dry day 5kv bias charge and on a humid day 3kv-4kv bias charge, I tend not to do any evaluations or serious listening on days when the humidity is above 70%. Or if I have to, I can with some trouble turn down the gain of the bass electronics by the appropriate amount so all is balanced again.
-- Al