How much difference does a phono preamp really make?

A good phono stage with loadings correctly matched to the cart (or to your taste if you prefer) will make a big difference, more so with a moving coil, more so at the high end.

Miller, please send me your 'worthless' used carts so I can get them re-tipped with replacement suspension and they will be like new.  Whilst some robbers, like Koetsu, charge obscene amounts for re-tipping (but still less than half the new cartridge ticket), reasonable people like van den Hul and the Garrott Brothers work at sensible charges.

Noob, maybe try a tubed phono pre.
Dont hear a difference you like?
Roll some tubes. One like the Manley Chinook for example, one that's offers many gain and loading settings to possibly be a good mach for a number of cartridges down the road.

A manufactures settings are a suggestion, use your ears.

I run a moving coil Lyra. It's a lively detailed cartridge but I wanted to run my phono pre at a lower gain to lower tube rush (noise).

Enter Bobs Devices SUT. A SKY 30 set at 1:30 per Bobs recommendation
(as mentioned above)

Wow, he was right. More of what I already enjoying and quieter. Love it.

Good luck in your hunt

I'd buy a Tavish Design the classic or the vintage tubed phono preamp, or maybe even the EAR 834 p. You will hear a difference with either.

The question is what phono stage, not all phono stages are good, not every higher priced phono stage is better than lower priced phono stage. Sometimes they charge for usability, but not for the quality. 

Tube phono stages are more problematic, because tubes are noisy, especially for low output MC cartridges. 

Solid State phono stages are different too, but they last forever, tubes are like cartridges - you can use them only for certain amount of time. 

Here's my take.  In a vinyl reproduction system, the phono preamp system, which could be simply a built in phono stage, a separate phono preamp or those in combination with an SUT, is the most critical part of a system's amplification chain.  This is because it's tasked with taking a miniscule output signal from a phono cartridge and amplifying it up to line level, while trying to act as a straight wire with gain.  This is a difficult task.

Phono stages in preamps may be good, but their mostly good for a built in stage.  A serious quality separate phono stage under most circumstances should improve your system.

But once this is accomplished, the rest of your system has to be of sufficient sound quality and neutrality to be able to discern the  differences between the various phono stages.  No part of the reproduction chain is unimportant.

The only way to tell for sure is to listen and compare, only you can judge using your own system what improvement a piece of gear give you, and whether that improvement is worth the cost of the component.

Good luck.