Yes indeed, sorry for my mistake. The readings are valid, though. 500 pF is what the transformer sees, as @maudio and you stated before. This was just a quick test to reveal any fault after my rebuild.
In a electrically segmented esl there is, so only the high frequency section is driven directly. That helps a lot. On the 57, this section is also driven from a lower stepup which makes it even easier for the amplifier. They had to, the 57 was designed when a 15W tube amp was top of the range.
Most designs without electrical segmentation also have correction networks, but on the primairy side (Martin Logan for instance). Some have both sides.
It's possible to model the impedance by transferring al secundairy circuitry to the primairy side, but the level dependancy of transformer parameters makes such models quite inaccurate.
I've never seen any mechanical changes to the stators on any of Wayne's panels that I've worked on. He was a big fan of painting EVERYTHING. I strongly suspect that the resonant point of the bass panel was slightly different from either a tension difference or the elacticity of his diaphragm material choice causing the difference. But a change in resonant frequency or Q of the panels can make a HUGE difference in perceived bass in a set of original quads.
I suspect that the original quads had more bass when new than an un-rebuilt set now due to the coating evaporating and the saran material aging.
Sheldon