Flooring advice on basement listening room.

The seismic vibration is continuous, micro seismic activity as opposed to earthquakes. Plus traffic, wind, subways, etc. The reason these relatively small amplitude low frequency vibrations are a problem for audio is that much of the signal, e.g., phono cartridge, tonearm wire, preamps, DACs, are low level signals and completely susceptible to low frequency seismic vibration. I.e., electromagnetic waves are distorted by vibration. Even high level signals are susceptible to vibration as we’ve seen with cable risers, etc. Thus, even for rooms on cement slabs vibration isolation improves the sound.

Don’t shoot me, I’m only the messenger. If you don’t know just say you don’t know.

No. That’s what you say I’m talking about. 

Whilst it’s true that cement slabs are better than wood floors since they’re much stiffer, you’re not out of the woods yet because seismic forces, including Earth crust motion, cause the entire building, including the cement slab, to move in all directions. Like a boat 🛥 is moved up and down and all around by waves passing under it. That’s why seismic isolation is beneficial even for rooms on cement slabs. When you isolate something you’re isolating it from the entire building and the motion of the Earth. 🌎