I would recommend a Audio Research tube preamp. This would be the next step in refinement and fidelity. I would recommend buying used and look in the $5K range.
Enthusiasm for this pursuit is key and admirable. The increases in fidelity are nearly limitless. I have been pursuing this for over fifty years and each upgrade during that time has rendered my system’s sound better and much more enjoyable. But to get the very best sound requires a great deal of planning and research.
You have done really well so far. I think after purchasing a great preamp I would spend a lot of time listening and learning before embarking on the next steps. As you move on you will want a well thought out game plan for future upgrades. I recommend Robert Harley’s The Complete Guide to High End Audio as a great place to get the lay of the land.