Ralphiano I wonder if what you are loving so much might be the "Aliquot" string that Bluthner has used in at least some of their grands.
I definitely suspect that to be a major factor in the whole attractiveness of the sound. Something that gets filled-in, gets rounded, gets completed that I miss in quite a few other pianos.
There is also that warmth, a hint of mellowness, and woodiness (am I describing a beautiful single-malt here? 😁 ) that I just love so much.
I haven't yet seen the Aliquot system up close, but I suspect the dampers (just like the hammers) don't/can't reach the extra strings so they'll decay and sustain on their own. They are excited/activated at a lot lower intensity than the actually struck strings, so wouldn't overpower or disturb following/changing harmonies.
From what I've heard so far, the 6--7' Blüthner instruments seem to be my favorites, for a touch more intimacy and self-containment than the 'concert' sized grands.
Cheers and happy discoveries,