a link to “decoupling”.
http://www.consumerelectronicsnet.com/article/Decouple-Your-Speakers-33664
I have one ~30cmx30cmx5cm recycled foam rubber pad (kinda soft but firm) under each monitor on DIY stands on the floor.
It’s just scrap material used for shock prevention for packaging fragile and expensive equipment.
I took it out of the waste bin, swept it in some black cloth and it’s perfect.