Which makes the biggest difference?

On my difficult to drive speakers the 4 amps I have had on them made a difference. It was not huge but on certain music the more powerful amps at 2 Ohms made a difference. I could also tell the sonic signature of the amps but that difference was not huge.

I have had a few DACs in the house, tube, warm SS, and neutral SS, and they made a smaller difference. The tube DAC softened things up while the SS amps were much closer in sound. When I added a Convolution filter for my very revealing headphones I could not tell the difference between the 2 SS DACs. The filter worked mostly on shaping the high frequencies. I also think DACs are of very high quality these days, with most sounding very close to each other.

The preamp I now use, a warm and detailed piece, made a huge difference over the very neutral preamp I way using before. Especially so in the bright headphones I use.

When I switched from Ethernet to Fibre Optical streaming I had the biggest jump in performance. It was not even close in my setup.

1) Streaming
2) Preamp
3) Amp
4) DAC

BTW - A preamp ALWAYS improved the sound in my setups. One of my DACs also does not have a volume control (a feature in my opinion and not a deficiency)

When I switched from Ethernet to Fibre Optical streaming...

Than you. i know the local ISP ran Fiber optic cable throughout the city/town where I live but hadn’t thought about it within the house. I didn’t know it was available. Now I do and will check further.

+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.