How much difference does a phono preamp really make?

As everyone else has said, the Phono Pre is as important as the Turntable itself because of the delicate signal it has to amplify. The hierarchy of the vinyl chain is Turntable, arm and cartridge in that order. With that said, I'm a little surprised that you can hear no differences  between the 2 carts. There should be some subtle changes. 

@ audionoobie OP

I can hear a difference between the two carts. Not so much with the two phono pre's
It seems like my carts may not be good enough for me to notice a difference between my entry phono stages.

OK, that changes the answer for me. Both preamps are about equal sonically so you won't hear a difference in them. But the carts are doing their job. You can hear a difference.  Though your carts are not really high end, they will respond much better with a better phono preamp. Remember the hierarchy I posted. The cartridge was the least important. 
Atmasphere noted as much telling how a lesser cartridge sounded great on a better arm & TT. The phono pre will also give better and is where I'd spend my $$$ now. 

IF The cartridge is the most important, then why buy a good TT & tonearm first ? Well, there is a reason why. IT AIN"T SO. IF the cart was the most important, people would build their vinyl rig around it. BUT THEY DON"T.

So, OP If you buy the hogwash of the cart being the most important, then you should sell your TT & stand alone phono preamp and buy a cheap TT and a very nice cartridge with those funds.

@ 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.