Which component(s) benifit the most from isolation?

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.  :^) 

Spikes couple directly to the floor.  Energy can go to the floor as well as come back.

I use speaker isolation springs. Mainly because they were $30 a set. I have Herbies under everything else.