How much difference does a phono preamp really make?

The most important component by far is the modem. Without that I am unable to connect to pay the utility bill, without which there is no power, and no system, and the power cannot even be turned back on. So it is definitely the modem. Or any other darn thing I can think of to make the point that there is no one most important anything. 

What you do noob, is if you have something that is known to be negatively affecting the sound, or is super easy or inexpensive to upgrade, then that is what you do. Because everything matters, and sorry to offend the others who got it wrong but everything matters pretty much about the same.  

Mostly what you want to learn is not so much which things make the biggest difference, because they all do. So forget about that. What you want to learn is which things can you do that will give the biggest benefit over the time you will have them.  

So for example a really sweet cart will be a huge upgrade. But depending on how many records you play it may last only a few to maybe a handful of years, at which point your wonderful cart is just about worthless. While a turntable or arm, good ones will make about the same initial improvement but last for years and years. Mine are over 16 years now. I have gone through three carts in that time. If I had bought the Koetsu way back then it would indeed have been a wonderful improvement. No doubt about it. It would also have kept me from being able to buy the table I got instead, which is now with me and running fine all these years later.  

So to really get the most from your audiophile dollar calls for planning the likes of which few here can truly advise you on. Once you have the plan, then we can. But by then you won't need much in the way of help. So relax. There is no right. Or wrong. And you will never be done- until you are ready to be done.... for now.  

@ audionoobie OP

I didn’t say that the cart is the most important component.

My apologies for the confusion. I know you did not say that. But others have. The lack of logic in the idea that the cartridge is most important is what I was trying to expose. Nothing wrong with your TT & arm. Yes everything matters also. But unless you have very deep pockets you have to prioritize what upgrades are necessary sound better. I seriously doubt a better cart is going to be a big upgrade. It may be some better. But you are still going to be wondering about that phono preamp.

1st, a new cartridge,

I like Audio Technica, however I would go with the same body with the Shibata stylus and save $250.

https://www.audio-technica.com/en-us/cartridges/type/moving-magnet/vm750sh

both have very strong stereo separation, 30db and tight channel balance 1 db.

I am using my ancient but still near virgin AT440ML upstairs, and got my first MC, AT33PTGII, very similar specs, main system, I love it.

Any advanced stylus shape requires very careful alignment to sound great and to avoid damage, how are your mounting/alignment skills?

The phono pre makes all the difference in the world....
Regarding the limititations to the cartridge and arms strengnths and inherently their weakness’s..
As well as its own strengths and limitations in regard to the previous’s real capailities....
Think of them as one importance in how they are relevant to each other....not how one is more relevant than the other.
The thing about the phono pre it can limit the strengths of the other two, but a good phono pre will show all their capabilities and flaws...
You don’t want the phono section to be the bottleneck if you have a good cart n arm . Conversly, if your planning a better arm or cart or both.....how will you know what they can do if the pre isn’t up to it..... this all hinges on the idea you know how and do have arm n cart properly set up.....

OP, as others have said the phono preamp makes or can make a material difference. It changed the game for me. If I didn’t jump to a good stage (Manley Chinook) I would’ve thought digital had eclipsed vinyl for good. That is not so. They are different but vinyl can be seductively mesmerizing.