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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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. 
When comparing different  or even the same ML ESLs, you need to consider the room placement and the partnering electronics. The model you heard in someone else's home/room will sound different in your room. Probably true for all speakers.....
@mijostyn -- thanks for the tip, but $17,000?  I'm sure they sound great, but that's even more than the ML 13's were; I was looking for something in the range of $2-3K.  I will, however, read Sanders' papers, and continue to dream...