Which component(s) benifit the most from isolation?

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. 

Sorbothane has no place in audio.

@jonwolfpell 

"Many don’t agree w/ me but I’ve found over the years w/ several different systems that if the listening room has a very solid concrete slab floor or similar, it’s best to couple your equipment to it & let the mass of the floor / earth do the job."

Very wise statement, no one worried about "isolation" it seems until hifi equipment became so good that hobbyists needed something else to worry about.

Personally, I prefer to couple as in spiking speakers to the floor and having a reasonably solid platform for my turntable that is coupled to an equally solid rack. As far as preamps, CD players, amplifiers etc., I don't really care!

Isolation is a great DIY project.....sorbothane, silicone, and rubber pucks and cutting boards are simple, affordable, and as effective as many commercially marketed.  I isolate my preamp, phono stage, turntable, and amps. 

My subs and speakers are isolated because they’re on a resonant suspended floor....coupling to that with spikes excited the resonance.  I would use spikes and couple them to a ground level concrete floor.  A suspended concrete floor can resonate too, so coupling to concrete is not an absolute.  Every situation is a different, so needs to be treated accordingly.