finding the weak link

What is the perceived deficiency? You do not say. What is it you are trying to improve upon? Seems like you just want to throw money at something...not a good thing.  

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).

@larryi : I agree with you! The 6L6 family is my preference for output tubes. I presently have a pair of Heath W5's and a pair of Stromberg Carlson's in use! Along with my Fender Twin Reverb guitar amp (black plate RCA's).

The biggest change the OP can have would be a different pair of speakers. How about some Ohm Walsh's? I have a pair of the Sound Cylinders and OMG do they fill a room with 3-D SOUND! The performers are RIGHT THERE in the room! Ohm in Brooklyn can build a pair of Sound Cylinders in any finish the OP desires!

Oh, I'd like to hear those speakers!