Which component(s) benifit the most from isolation?

Phono pre could be the most sensitive. 

My Aesthetix Mimas and my Manley Chinook each employ tubes...

The idea of isolation is to prevent shunting from interfering with process, every device is different and so needs considered isolation.

I got round the problem by using 32mm starlight quartz platforms cut just over feet size and even when on carpet using non slip matting underneath.

It may not isolate as one expects but it sure makes a best positive difference, even when the item is on the floor.

Just don't buy anything expensive without the possibility of return for a full refund. This is a complicated matter, and maple blocks with brass cones might be all you need. And it might not. Coupling/decoupling can work together. Trust your ears.

Ya, the tubes first, phonopre, then the dac. You can always try sorbothane pucks from Amazon for a cheap stop gap.