I've never heard the Thiel 3.5s (that I recall) but used to listen to the 3.6s here and there, though many years ago. So you have to take any comparison I'd make with a big grain of salt. That said, I also lived with the big Thiel CS6 speakers for a while too, so I have a pretty good bead on the Thiel house sound in general, I think,
Whenever I listened to the 3.6s "back in the day" my impression was always consistent: a sense of accuracy, in terms of tonality, soundstaging, imaging and a sense of life - great transient quality and an overall sense of control. There was a real confidence, a sensation of "hearing the recording for what it is."
The slight knock on the 3.6s was, as many have said before, to my ears a teetering toward brightness and a tad bit of hardness. Just a tiny bit of a "ruthless" quality. Not as bad as that sounds, but just tipping a bit in that direction.
The other is a slightly "reductive" quality that I tended to hear in Thiel speakers. The density of the imaging had the great result of palpability, but it seemed the Thiels could squeeze the sound just a bit too tight, with a signature that to my ear could seem to slightly remove the amount of body and heft of the sound of voices and instruments. (The smaller woodwinds, for instance, could become fairly thin sounding).
The CS6 speakers I had, while sounding huge overall and full from the upper bass down, also had this slightly reductive quality. Though I found them not bright at all, and not in to the ruthless territory so much. They sounded gorgeous with my CJ tube amps.
The difference I find with the last flagship 3.7 (and to a degree the 2.7) is that I do not find any of this reductive quality. Voices and instruments sound smooth, big, even and as lush as one could want. If you have an acoustic guitar recorded up close it is BIG and full. The sense of image sizing and soundstaging goes beyond any Thiel I had heard before. And the tonal balance seems completely smooth - missing the slight hollowing sensation I could occasionally hear in previous Thiels.
And they have a more open and delicate way with fine detail.
While giving you that aliveness, snap and image density and specificity that Thiel is known for.
So to my ears the 3.7s take the Thiel sound and simply refine it.
As I've pointed out in my other, long speaker audition thread, despite listening to many of the best contenders now available, I did not find any that seemed to do it all as well as the 3.7s.