Differences among the electrostatics of Martin Logan models?


I recently heard a pair of Martin Logan ESL13's (Expression), and loved them, but they are way out of my budget.  I understand the electrostatic portion of the ESL 9 is nearly identical, and that got me thinking about pairing one of the many models of used MLs (eg ESL X, EM-ESL, Ascent, Sequel, Scenario, EFX, etc) with a good subwoofer.  Does anyone know which models of ML use the same electrostatic section as the 9's and 13's?  Which older models would have noticeably inferior electrostatics, and which should just be avoided? 
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Thanks ledoux, I agree with you; the X is not as good as the 9; unfortunately, the 9 is out of my budget too, so I'll have to save up for now.  Good luck finding your ESL 9's!
If you can get them with good panels, the Monolith III was a great speaker. I have the latest panels in mine and they can't be touched for sound quality.
I always said if I didn't have these a used pair of Summits would be on my list for a more manageable (waf) speaker..

Cheers George
Figure the cost differently....mine are 8 years old...bought them used.....Figure they last you 12 years....divide the price you are looking at by 12 and that is your cost per year....about the same as a new iPhone....
@palmharbor -- Thanks for your response, I think that's a good way to approach it.  Unfortunately, I listened to several pairs of used ML's, and none of them sounded as good as new ones.  I even listened to a pair of Sequel II's that the owner said had new panels installed 6 years earlier, and they did not sound good.  After trying to find a used pair that sounded as good as the ones I initially heard, I eventually bought a more traditional set of speakers.
You might want to check out Sanders ESLs. He designed the curved panel for ML, left and created his own company. He is an up front guy.
His speakers are significantly better than any ML I have heard. You can not take a curved panel very low before they become completely non linear. Read Sanders white papers they will give you a lot of insight into his thinking and the world of ESLs.