Dear Radical, I had the impression from your first description that your SP10 plinth was made of some kind of metal, which is why I suggested you try wood or slate, not because I thought the mass was too low. So, what is it made of, in fact?
Raul, you and Albert may have arrived at the same endpoint in different ways. In Albert's wood plinth (which can be seen on the Sound Fountain website), he employed a steel rod that is threaded so it can be tightened against the bottom of the SP10 chassis. That rod is also attached to a dense metal block at the base of the plinth, so as to drain energy from the SP10 chassis. In a way, that achieves the same end that you achieve by sitting your chassis right on the three feet. I am trying to figure out how to do something similar with a slate plinth.
Raul, you and Albert may have arrived at the same endpoint in different ways. In Albert's wood plinth (which can be seen on the Sound Fountain website), he employed a steel rod that is threaded so it can be tightened against the bottom of the SP10 chassis. That rod is also attached to a dense metal block at the base of the plinth, so as to drain energy from the SP10 chassis. In a way, that achieves the same end that you achieve by sitting your chassis right on the three feet. I am trying to figure out how to do something similar with a slate plinth.