I went through a very similar decision-making process. After auditioning 1.6's, 2.4's, and 3.6's, I preferred the CS3.6's (yes, even over the CS2.4's!), although the CS1.6's with the SW1 sub was pretty darn nice sounding, too. What I like about the 3.6's is that they are a bit more full sounding, and they can convey a sense of "weight" that the 2.4's couldn't match. Yes, the 2.4's (and 1.6's) are even more focused and precise in the upper mids, but to me, it didn't sound as musical as the 3.6's (which admittedly sound a bit veiled by comparison). But playing some big orchestral works on the 2.4's, and then hearing the same piece on the 3.6's really convinced me. Plus, with the 2.4's or the 1.6's, I would definitely need a subwoofer to get a level of low end response that is satisfying to me, whereas the 3.6's don't necessarily require a sub, with the right amp. In the end, I bought the CS3.6's, and have been very happy with them, even though my room is not very large, either.
Speaking of amps, I also own a GFA-555II. Two of them, in fact. I had been driving the CS3.6's with each 555II in bridged mono, which is a boatload of watts, but which has the potential to present the 555II's with a difficult load to drive. However, the Adcoms run cool and clean with no problems. Yes, this setup sounds very good. I am using a Rotel RSP-1066, which isn't top notch, but I feel is quite good for the money. With all that being said, though, I am presently considering upgrading my amps, and most certainly will do so. I am hoping to A/B my Adcoms directly against Krell, McCormack and Theta, but Pass, Bryston and Classe' are also on my "short" list. The Adcom GFA-555II is a fantastic amp for the money, but I do think that Thiels in general, and the CS3.6's in particular, will reward efforts at upgrading the rest of your system. They are really quite capable speakers.
Being a Thiel fan for years, I would be inclined to think that you would be satisfied with either the 1.6, 2.3 (although 2.4 is a significant improvement) or 3.6. With the 1.6 or 2.3/4, though, a sub is necessary, IMHO. And with any of their speakers, an upgrade from Adcom components would yield definite sonic improvements, though in the short run, I think that they would sound fine, and by the time that you save up for an upgrade, you will probably be in a good position to appreciate the sonic improvements.
Good luck, Tom.