How much should I pay for a 2001 Thiel CS 3.6?

I'm an NAD fanboi but you are better off with a MUCH MUCH higher power amp and upgrading to a more refined high power amp in the future. Some of the Adcom 555/5500 series will get you a lot further than a new shiny 80 wpc NAD.

Reason I say this is that I used to own Thiel's and they really needed grunt to sound right. The only way you will get these right, just as others have said, is to power the crap out of them, and that means into a 4 ohm load.

Forget 4 Ohms, how about 2.3 Ohms!

Even 250 watts as a stand alone spec won't necessarily provide what is needed to power these properly. Trust me, I own them and love the sound of them, but it took a Krell 400 cx (even a 300 cx wouldn't do, I tried!) to finally get them to "light up" and pump clear, textural bass as they were designed to. Also, you need current to get them to warm up and sound sweet and delicate when called to do so. When you get the power right though, they are wonderful speakers.

An amp that doesn't flinch into 2+ Ohm territory is de rigeur.

I've had my 3.6's for over 14 years and have been driving them with a McCormack DNA 1 Deluxe that I had upgraded by SMc Audio to Revision A Gold with some extras in 1998. The amps are rated at 185 watts. I have to say that I have enjoyed them very much and I do wonder what they would sound like with a more powerful amp in consideration from the comments here. I do find them to be somewhat bright at times. I have been strongly considering changing the amp and letting the Thiels go in favor of the Vandersteen 5A's. Not sure when that will happen.
It is true that the tweeters are not in stock, however mine had to be replaced recently and I was able to send it back to Thiel and they rebuilt it. Good as new and they were a pleasure to work with.

i have a single thiel  3.6 coherent loudspeaker looking to complete the pair, who's got the other, please contact me.