Thiel CS 3.7 and Vandersteen 5a, 5a Carbon

Crutchfield has a 60 day return policy. Should be sufficient time to run in and see if you like it.

I upgraded from a 2.4SE to a 3.7 and have had no regrets. Running in was also less taxing than on my 2.4s which were hard to listen before I had 200 hours on it.

If you're enjoying the 3.6's in your current system, I wouldn't look at Vandersteens. Both are outstanding speakers but at two different sides of the spectrum; Vandersteens tends to be slightly dark and Thiels slightly bright, neither to the extreme. I would think that you probably have a system that tends to balance the Thiels, which means that with the Vandersteens everything will be too dark.

Thiel 3.7 is a major step up from the 3.6's.

I own both Thiel CS6 and CS2.4 as I am a huge Thiel fan, and I have listened extensively to the 5a. If I had the bux I would go with the Vandersteen simply because the bass alone is unparalleled and I do not find these speakers to be dark. They are simply amazing.