many thanks for the reflections and contributions. I am realizing again that the sound of a grand piano is still ill understood - no consensus at least on this forum, and even if there is probably a lot more literature out there somewhere, there are still many questions to answer. And we certainly do not need to understand it all to fully appreciate beauty.
It would be useful to do a spectral volume analysis of Leon McCauley's piano and others, with and without acoustic insulation. An academic paper describing the results would be very interesting. I think this would be a good basis for the research into rim and bridge termination materials William proposes.
PS This post is intended to end the hijacking of the OP's thread and bring it back on track !!!
Thanks for this clarification. Good to know also that this has not been researched according to the people here. Certainly this sounds like a lovely project to take up at some point.
I would personally be surprised if the growl of a grand piano depends largely on acoustic coupling to the legs or the rim - I would personally have guessed that the strings and soundboard would a primary cause for growl, knowing that this is after all the primary resonator (and seeing the data even if only partially understood in the TUE paper). I am curious to hear the effect of decoupling to see if I can hear it. If I can visit the UK - I definitely will and go an listen as the post by Ian suggested.
To indeed direct the discussion back to where we started, the comparison of some great instruments and the discussion on a growling bass: do have the impression that growl makes it easier for the piano sound to be bigger (not sure it translates into loudness, but clearly there is subjectiveness).
In any case, I can report that I am still exceedingly happy to play the 280VC. The Bosendorfer does not growl, but it certainly can make a lot of noise when I want it to make a lot noise. I very much appreciate the finer levels of gradations that a top instrument can make. That fine control of tone makes it much more challenging to play cleanly since I can hear much better what I am doing wrong. I am glad not all audiences are as critical, but it certainly helps my own technique.