How would you spend my next $5K?

@hysteve Thanks.  I got a both USB and NET used, elsewise would not likely have sprung.  NET seemed to make a bigger difference although that may be because I got it first.  Deeper blacks. A more “settled/relaxed sound” and somehow a better grip on bass though I don’t know why that happened.  Innuos says  NET+USB get Zenith MK3 to 90% of previous pre Next Gen Stmt.  

Room treatments are what 90% of audiophile systems are severely lacking.

Otherwise, I recommend you spend the $5K on contact enhancers, fuses and voodoo shaman chicken bones. 

A second sub made a huge difference in my room so that would be my recommendation 

@helomech      +1   The issue with room acoustic treatment is the difficulty/expense and aesthetics that can prove overwhelming to many Audiophiles. Look how many "Agon systems" have that dreaded large flat screen TV prominently showcased. Since the OP never mentioned room treatments or dedicated power lines that would IMO make the most improvement.

Room is the issue for me to add components, but for the money, eh, maybe a bit more, a Naim Uniti with a 20 terabyte drive.  A local dealer that sells it would include his entire library, 16 terabytes of hi def music.  Music of every kind mostly ripped from the thousands of LPs they have and a host of other things that would take too long to describe.