Do you like the sound you are getting? Are there certain things you don't like as much and want improvement? What have you heard out there that you tempt you in that direction? There is no easy answer to where to go net in a system that appears to be fundamentally sound.
If it were my choice, I would be looking to replace the Prima Luna--it is not my kind of tube amp, and all my gear, except the music server, are tube-based. This will require you to go out and listen to alternatives. Another possibility, if this is one of those Prima Luna amps that allow the use of a WIDE range of tube alternatives, is to try different tube types. My personal preference in pentode/tetrode tubes is the 6L6 or KT-66; both deliver considerably less power than the KT-150, but I like the sound a lot more (less "glare" or brittle quality to the sound).