How much difference does a phono preamp really make?

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. 

First, in any chain of events the first stage is the most effectual in performance, all errors are magnified from that stage on.

Second, the relationship between the cartridge and the phono electronics is as tricky as that of the amplifier to speaker, each involves the interface between two energy converting transducers, mechanical to electronic for the phone, electronic to mechanical for the speaker.  There is little that is linear in either process.  

Cartridge loading sets the cartridge performance and is not a fixed issue, It effects the entire electro-mechanical relationship.

I would suggest that you read Austin Audio Works paper on cartridge loading (on the web site) to expand your understanding of this reality of this relationship, no fluff, just raw data delivered in graphs.  You have to decide whether the timbre of the cartridge is effected by what loads it.

Resolution of audio information starts at the beginning, and each record is different - check out the Black Swan.

@cakyol

Spend about $1000 MAX for a cartridge and maybe at most another $500 MAX for your phono amp and you will be set.

sorry but i don’t agree with this, just far too broad a generality, esp. re $500 for a phono stage

depends so much on the system and demands of the listener/user

It’s impossible to "read" the groove correctly with a conical stylus (and elliptical stylus is not the best), if you can’t pick up the information (music) from the groove correctly then no phono stage will help you, no matter what price.

1) Vinyl Record is the source of music.

2) Cartridge (stylus tip) is the one and only device that actually touch the record and "read the groove" (read the source).

Everyone should have a great cartridge first and then everything else on equal level.

Always start with a cartridge, not vise versa. This is the first and most important component in analog chain.

** read and watch this and that at least.