Andy,
My ears agree with yours.
If I had to generalize from what I hear in the Thiel first order upper frequencies vs many of the high order speakers, I’d agree with what you wrote: perfectly integrated in the Thiels. I simply can not "hear out" the tweeter at all. Where I often feel like I hear a little power/frequency dip in the transition to a tweeter, giving a slight pinched quality to the high end, on the Thiels the sound remains "thick and smooth" all the way up through the high frequencies. Where the best of the high order speakers do have a little bit more sparkle/shine and perhaps smoothness riding on top. I admit I can like both depending on mood.
Same with tube amps vs SS. Though I’ve tried various tube gear, by far most of my experience is with my own Conrad Johnson stuff (originally an MV55 amp, then the Premier 12s, along with my CJ Premier 16LS 2 preamp).
Every time I do an SS vs my tube amps test, one of the things that stick out is how the sound becomes more coherent and believable with the tubes. Vocal sibilance for instance which can take on a hardened electronic spikiness with an SS amp just seems to soften and sit back in to the voice in stead of stick out artificially, so then voices sound natural.Even when there are artificially enhances sibilance and transients due to microphone choices/mixing etc, the sound is still more comfortable and natural sounding. It almost feels like going from a not-quite-yet-integrated tweeter/mid driver to adjusting the crossover to a seamless presentation. (BTW, not that the Thiels in particular need tubes to be smooth and integrated. I think they are one of those speakers that is successful with any reasonable amplifier).