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?