What to upgrade to?

Which amp do you have? 

I think the amp and the speakers will have to match to work really well. You could change both at the same time and sell your old amp but that might leave you less money for the speakers.

I also think the speakers will make a larger difference but if you are satisfied with your system you don't really have to do anything. What you could do is to find some used speaker or amp (or TT) you are curious about and buy that. Try it out for a few weeks and then sell it if nothing improved.

An amp that impressed me was the Abosolare Integrated but then you would have to let go of your preamp also. For speakers I like a number of Focal speakers, the Sopra 3 should be around $20k. The Boenicke W11 SE is also around that price.

Bob perhaps try reaching out to Vu at Deja Vu and getting some cartridge recs that match up well with your phono pre. Vu's given me tons of great advice over the years he really knows good sound.

Jond,
That's a great idea!  Why didn't I think of that!  lol

Maybe you can leave that money on the side and use it when something in your system fails. For now, why change the team that is winning?

OP.  I built an isolation stand for my vintage audio linear turntable.
step 1 go to grantie shop and purchase appropriate size remanent cost $10
step 2 go to local building supply store and    get a 2x2 chunk of mdf cost $10.
Step 3 find a yoga mat that no one is using cost $0
Cut the materials so they all match in size
sandwich the yoga mat piece between between the granite and mdf use construction adhesive to adhere them.
clamp it together and wait for the adhesive to set.
this works about as well as my symposium shelf under my vpi scout.
point is have fun if you're retired may as well have something to do, exercise your brain figuring out how to do it.