Why nobody uses McIntosh with Vandersteen?

Theo,
Just a guess--the Vandersteens have a very pleasant, forgiving sound (not really my words but those of many who I've attended shows with), and MAC has the same sort of sound. So the thinking might be that together they would not be revealing enough.
While I can understand that basic thinking, I don't necessarily agree. Tubes are considered softish,but more revealing perhaps than the Mac Solid State, like the 6900.
As to the Autoformers, I couldn't say, even though I attended the McMasters Training in their bustling metropolis!lol
On the taps for the loudspeakers, I know exactly what you mean, it makes one wish there was a 6 ohm tap.
THIEL's sound good with MAC--as they ARE forward to some folks, (not all, for sure). It may be, as you said a classic mismatch.
This is one reason why, in developing loudspeakers, I used the Halcro (very low, low, low distortion, but in some opinions, dry (I've even heard some say amusical), then Grypon, then a 10 year old Receiver from Harman Kardon.
Admittedly, I didn't try the Mac, it may be that it's just 'soft' enough to be a mismatch.
The good news is, you've got lots of options.
Good luck,

Larry

"Mac owners use B&W, JBL, Tannoy, Sonus Faber etc"

McIntosh MC501 monoblocks with Wilson Sophia II's here (4 ohm), no thin problems.

Isn't the "feature" of the Autoformer is to make the amp is compatible with more speakers?

Yes, but Macs don't have 6 Ohm output.

all McinTosh with Thiels

My brother has a Mac and Vandy system. Sounds pretty good to me.