Which component(s) benifit the most from isolation?

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.

The cement floor is coupled to the earth. The earth moves. My audio room has been on a cement slab most of the decades I have been an audiophile. Cars, trucks, lightning strikes, workmen, nearby footfalls, small movements in the earth. I own a seismometer, it is easy to discern the day from the night on it, from the background noise. The first day I owned it there was a boom from a lightning strike about two miles away... wow, did the concrete slab shake.  I have tried various vibration control (pucks, inner tubes, cones of different materials, platforms... etc. ) on my systems over the last fifty years. One of the most shocking and substantive changes was to take my 135 pound speaker (each) off of spikes and float the on Townshend Podiums. A shocking improvement in sound. Also, I know of a lot of folks that have done this, and no one has ever commented that it did not produce substantive positive improvements.

I put a seismometer on one speaker and played the other when still on spikes. It was shocking how much vibration was picked up from to other speaker.

I have a Linn LP12 near Klimax. So a sprung... spring isolated table on a spiked audio rack. The improvement in sound when I put a custom made Silent Running Audio Ohio class  isolation device was very noticeable and positive... the background got much quieter and images much more distinct... better bass. 

While coupling to a concrete floor may be better than coupling to a suspended floor in a noisy area... it is not an effective vibration reduction strategy.