Upgrading from Thiel CS3.6 to Avalon

I don't believe it is an upgrade. You will lose the phase and time coherence and you will know it when it is missing. If you're at all like me, when I moved away from Thiel to Dynaudio for a couple of years, I moved back because I missed that pinpoint positioning of instruments and accuracy of timbre that the Thiels furnished.

Why not truly upgrade to 3.7s? Or Vandersteen 5As?

I went from 3.6s to CS6 and to Salksound Soundscape 8. I am very happy with my latest purchase. IMO, The Salk is much better in every way.

I've owned the CS1.5, CS2.2 and the CS2.3; you will find out that the Avalons' are much smoother, very coherent and projects a more stable image/soundstage. No forwardness or hardness in the midrange area YMMV...

IMHO, Avalons' are superior...

Overall, I have to agree with Stevecham. If you gave the Avalon's a thorough demo, and you know you really like them, that's fine. Otherwise, I wouldn't buy any new speaker. The ones you are looking at are just too expensive to take a chance on. Also, I would consider the 5A's, as well. Not surprising because Vandersteen is my favourite speaker.

Tom-

Thiel speakers are a unique beast. Upgrade to the CS 2.7 or CS3.7 model.

Keep me posted on your decision.

I owned Thiel 3.5s for ( 8yrs ) Loved Them! Drove them with Forte' monos (Nelson Pass design) and Spectral electronics. Thiels; at least the Legacy models are as you know, as an owner, are what everyone claims. Very hard to find a speaker as good or better for the money. I tried Pro Ac, Wilson, Sonus Farber but finally settled on Avalons. Opus w/ carbon woofers. The Eidolon come up for sale fairly often, I have never seen a pair of Opus for Sale. The Opus will fit your room well ( my room is 13w X 23L  X 9h ) My opinion- The Avalons are superior. Better imaging / soundstage. Smoother top to bottom, no need for a subwoofer. wigs comments also apply. The Eidolon; I heard both at the dealer many times and (6wks) in home demo. But I settled on the Opus. They will fit & fill many rooms. Your equipment should mate well. Now that Jim and Kathy are gone. ( even though Thiel seems to be supporting the Legacy models ) I think you will find the Thiels to be getting long in the tooth and service to become problematic in the future. 
If you can find a pair of Opus w/carbon woofers ( ceramic lack in the bass) You will never look back.

Best Wishes on your Journey.