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.

I disagree that moving from Thiel to Vandersteen will mean "everything will be too dark". I had Thiel 1.6s. I really liked them alot but wanted more bass as they pretty much didn't have any energy below 50 Hz. I considered Thiel 2.4s but decided to go with the Vandersteen 2Ce Sig IIs. IMO, the Vandersteens are better in most ways, with a full, musical sound the Thiels couldn't match albeit the Thiels wer more transparent and, maybe, a smidge more dynamic.

I've never heard the 5s but have heard the Sevens, Quatros, and Thiel 3.7. These were all in different systems/rooms, so I will not offer comparisons. They all are great speakers, however, and there is no way to make a mistake with these choices. The Sevens are a top three all time speaker in my experience. If I could afford the closely-related 5A Carbons, I would get those. I think that Vandersteens, on balance, are a "better", more musical speaker than Thiels. That said, the 3.7 is a great speaker (that also looks much nicer than any Vandersteen) that I could happily live with.

I think the 3.7 bass response is just not over emphasised and with an upgrade to Electrocompaniet Nemos (1200W into 4ohms), I thought it really came alive. I don't think it's power per se (my last amp was a 1000W into 4ohms Bel Canto REF1000M).

Anyway, I went with dual SmartSub2s to match the 3.7s which extend the bass further down to 15/20Hz.