The chest is old and crudely made, with a very thick top, and seasoned since sometime in the 1600s. But undoubtedly, you are right, @pindac, there must be effects of percussing a drum-like surface like the top of it. Given that the top of the chest must vibrate somewhat on being struck, how can there be little or no sound revealed by the cartridge? I assume the fact the pods are sitting on spikes is helping decouple them from the surface - a mechanical isolator rather than one resulting from material science.
But I suppose the practical question is whether my ear can hear the resulting vibrations as transmitted to the cartridge, and I’ll be the first to admit my hearing is compromised, having one ear with about 50% sensitivity in it, and the other totally dead. So if I can hear nothing through the cartridges mounted on the built-in mounts on the percussion test, and nothing through the Grado on the pods, I can say that it works as well as I need it to do. The Soundsmith though does give a faint noise in the speakers, presumably because its cantilever and iron is lighter than that of the Grado (I’m making that assumption because it is a low-output MI, and the Grado is not. Probably most of the reduction in output comes from fewer turns in the fixed coils, but that alone offers no advantage over a high output MI: Peter Ledermann must be using lighter iron on the cantilever as well to make his "MIMC" cartridges because that is what makes them sound different). But this is with the pre-amp turned all the way up, and I would typically play an LP with the volume set to 15-20% of maximum. At that level I can strike the chest and hear nothing. Even so, I think you are right, and not just on theoretical grounds, because the Soundsmith sounds different to the way it sounded on the built-in mount. The Soundsmith initially was a disappointment to me when I bought it as a candidate to replace the Decca. It was so neutral and uncoloured (qualities that actually make it a very good cartridge) it did not get my feet tapping. I found I could improve my response to it with various resistive loadings—lots available on the NuVista Vinyl—and then discovered it sounded equally pleasing via the SUT. But it is a different beast on the bipod; there is more bass and this must be related to the new mount, and that must imply some vibration feeding back into the cartridge. Anathema to some, but I like it! If I am 'making a virtue of necessity' as the old phrase had it, so be it.
Thank you for your detailed response, as always.