Which component(s) benifit the most from isolation?

@zlone 

"I think I may have asked this before, but is there an economical and accurate seismometer you can recommend? My house is firmly attached to the Laurentian Shield, it would be interesting to see what is going on in all that rock."

LOL! I was considering responding similarly.

Concrete is a poor dissipator of Amplitude. Any energy transferred to concrete is to become a not too different amplitude when passed directly in to attached or other local structure.

Coupling directly to concrete will enable ambient energies being received by concrete to be transferred straight into a Support Rack for Audio Equipment. 

If the Support Rack is not designed to counter such energies being received, the Audio Equipment will receive directly similar amplitude.   

I stack everything on top of the speakers and let them fight it out.

@mapman All of it to various degrees. YMMV. 

+1

Seems all with electrical circuits.

Substantial wooden platforms and racks offer a somewhat affordable isolation option, maple being a popular choice. 

I stack everything on top of the speakers and let them fight it out.

Good one!  Definitely puts you in a good place for further improvements.  :^)