Which makes the biggest difference?

+1, Fiber optical conversion in house big improvement. In fact the whole house network extremely important for streaming.
I see the amplifier as separate from digital, preamp important only because it's volume control may be used in place of dac volume. Match amplifier to speaker, not to dac. I'd say preamp and dac equally important.
In my order of preference: Best is dac to quality active preamp, next dac to integrated with passive volume control or stand alone passive pre, next dac straight to amp, last dac to cheaper active preamp. Colorations introduced by cheaper active preamp is least desirable.

The DAC!

They all make a difference obviously, but the preamp is the hardest thing to get right and to me exerts the biggest influence overall.

@artemus_5 Here is a link to how I got going with Fibre optical streaming. I ended up with 2 Sonore OpticalRendu’s. I bought the second 1 immediately after listening to the fist one. I have 2 DACs. I am also a big ROON user so ROON READY was a must for me.

https://www.sonore.us/systemoptique.html

I did not setup my streaming exactly as described above. I used the following network switch instead. Which has 2 Fibre slots.

https://store.ui.com/collections/routing-switching/products/unifi-switch-8-150w

A recent lesson I learned the hard way was not to put the ROON Server on a computer connected to the home network via a PowerLine network, put it on the regular Ethernet (in any room). With high res music I was getting drop outs which made my think my speaker drivers had gotten damaged. It was with George Harrison’s 50th ALL THINGS MUST PASS. On the song HEARR ME LORD my drivers would sound distorted during busy home internet traffic times during the passages when he is humming. It only happened on that song and was reproducible the times it happened. Luckily I figured out it was the streaming before I took more drastic measures with the drivers.

I find that fundamental choices for amplification make the biggest differences among electronic.  That choice must be compatible with the speakers that will be used, listening level that is preferred, and size of the listening space.  There are substantial differences between solid state and tube amps, and even bigger differences between high powered tube amps and low powered tube amp.  I happen to prefer lower powered tube amps because I like certain low powered tubes, like 2a3 and 45 triodes, and 6L6 pentode tubes.  But, if higher power is a must, I would consider solid state.  The choice of preamp is also critical because it has to be compatible with the amp.  This can really only be known by actually trying them together.

Assuming one has a reasonable set up to feed the DAC (i.e., a computer or streamer isn’t screwing things up), you can get decent sound from most DACs and the choice of DAC is usually not as critical as the choice of amp or preamp.