Tomthiel,
I agree that the CS3.6 is the model to start with. There were so many sold, and they have had so many accolades, positive reviews, and historical references, that people who own them just love them, and people who dont, do remember them. There is nothing in the used marketplace, even close to thier audiophile sound qualities for the prices they sell for now. So once you get a couple of thier sonic flaws eliminated, and the word gets out that Mr. Tom Thiel (and Rob of Coherent Source) offer mods for them, one of the USA's most famous speakers (ever) can become a truly collectable "classic" item.
I will stick with my Thiel 3.5s and 3.7s, driven by MFA and ARC 6550 tube amps (respectively) of course. BUT, I do have a BIG solid state amp, that I'm not using, just PERFECT for 3.6s: Classe DR-250, 250 wpc, and one of the sweetest big amps ever made.