Here is a trick to improve the sounds of the sub-woofer, nothing new though, I didn’t invent it but tried it today and it really tightened up the bass and (hopefully) results in less disturbances for my neighbors below me.
Basically you want to separate the sub-woofer from the floor, but still provide a very stable foundation. To do this, I bought a concrete slap, like this (one side is polished, which I put facing up). The width/depth is similar to the sub-woofer, and its 7 cm thick and weight:almost 30 kg (which is about 3 inch thick and 65 pounds):
To stop the vibrations in the concrete slab to go into the floor, put the concrete slab on some dampeners typically used to put a washing machine on, i.e. very solid rubber:
So to sum it up, from bottom to top you will have heavy duty dampeners, concrete slab and sub-woofer. Don’t put extra dampeners between the concrete slab and sub-woofer though, that defeats the purpose.
Thats it, the floor next to the sub-woofer has no vibrations now, and the bass became tighter and faster (easily noticed in my case). The cost for this? Depends, but for me it was about $20.
This might even work for big full-range speakers, but I haven’t tested it.