Starting the experimentation is the key, finding a material of permutation of materials that are perceived as a time well spent and a noticeable improvement over the previous used will be the reward.Great post!
Your experiment correlate with mine to some degree...
I used what i called a "sandwich" under my speakers which are on my desk and the same sandwich with dac and amplifier...
Quartz feet,granite plate,sorbothane in between, another granite plate, cork plate, bamboo plate...
This improve the sound quality...
BUT,
It was not enough....And i was not conscious of what was missing..... 😊
I used then 2 set of 4 boxes springs, one set of 4 under the speaker over the sandwich, another set over the speaker, under a fine tuned damping load of concrete ( 78 pounds)...
The essential point is the application of a dyssimetric compressive force between the 2 sets of springs which are then compressed with a different load....This decrease the negative effect of resonance of the rectangular enclosure....I was inspired by the " building tuned mass damping" principle....
Results : Amazing....On all acoustic count, timbre, bass, high frequencies....
Cost: under 100 bucks....
I had another idea about this to improve it , i must wait for another experiment in the week to come....
I dont think that those who cannot afford costly method must renounce isolation.... Isolation control is mandatory....