How much difference will a phono stage upgrade make?

The RCM is not tube based but as is the case with most quality components these days, the sonic differences between tubes vs. solid state is less than in the past. One thing I considered when debating tubes vs. non-tubed phono stages was the necessity for very low noise tubes in phono stages (and the expense of procuring/replacing them.) My experiences with tubes aging and getting noisier with the passage of time led me to prefer using tubes in preamps and amplifiers instead of phono stages. Perhaps those with more experience than I can weigh in on whether they have more noise issues with tubes in their phono stages vs. other components that amplify voltages to a lesser degree.

I too used a Primaluna Integrated with a Tube Box DS.  I made a smaller jump with HUGE results, upgrading to a Musical Fidelity MX-VYNL.  

In the fairly near future, PS Audio will introduce the Stellar Phono stage, targeted for the low $2000s.  It sounds like it might be a real winner, and all PS Audio is easy to audition in home for a month.  Just a thought.

Oh yeah, the Herron gets pretty good marks from everyone who owns one! Had mine since late last year. Easily the best component I've ever had. Or ever will, probably. Turns out there's a reason why they hardly ever come up used, and go in a flash when they do. My search turned up several people who had heard the various Herron versions go head to head with all the much more expensive supposedly best stages- even the earlier 2, even the original Herron, total match for anything out there. I don't want to diss the others, at this level there are no bad choices. But I did come across several people who said the Herron was better than ... fill in the blank.... and then added, and that wasn't even the current one! So you could get a used one (if you can find one, seriously, not easy- Itried!) and be very, very happy. 

But, why buy used? Why not just give Keith a call? I'm sure he'll be happy to build you one. He builds them to last, with fanatical obsession to detail.

How much difference can it make? Might want to check out my review. My wife heard it through a closed door! Cannot recommend enough. 

Before you buy another preamp would you consider rolling tubes? That's another avenue to explore.