How much difference does a phono preamp really make?

I think what most are trying to say is that it’s a matter of limiting factors. Look at your signal path and identify the weakest link. Figure out where you want to land for the system (usually budget driven decision) and upgrade the weakest link first. Any sort of smear in the signal chain can break up the soundstage - and no equipment can put it back together.  We have the same TT and I’ve had success with a 103R, a 1:40 SUT (Bob’s Devices) and a Decware tube phono preamp. I’ve since moved on from the 103, but it’s a great performer for the cost. Like most have said already the phono pre is important because it is responsible for more gain that any other equipment. Combine the high gain with the only tone control typically used in analog (RIAA correction) and you have the easiest piece of equipment to get wrong. 

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.