The sub boxes with woofer weigh in around 200lbs. You cannot feel anything form the subwoofer movement with nothing on top of it.
Why? They are constructed from two, one bonded pieces of heavy mdf. The boxes unloaded weighed in excess of 150 each.
Then there are two pieces of treated 4x4 stacked to hold a MANGO TREE TRUNK. The mango tree trunk was sliced vertically. It took three people to position the mango tree trunk on top of the 4x4’s that are on top of the boxes. I did not weigh the tree trunk, but I am guessing it is in the 500 lb plus range.
I can assure you the tubes on the 125 lb amps get more vibration from the movement of the air than transmission through four different density woods and metals.
The only equipment actually sitting on the sub boxes are a tape deck, CD player and the sub amp. The former two are rarely used as a source.