Keep McIntosh C2500 pre-amp or upgrade to C12000?

Try changing your tubes to a different brand/NOS first. I don't think your going to see that big of a difference moving to the 12000 from the 2500, better DAC of course. and the option for SS or Tube stages if that's something you like. I did a comparison from the 2700 and 12000 and the difference did not justify the price difference to me, tube stage comparison only. 

Tubes made a good difference with my 2700 when I was tube rolling. definitely able to tailor the sound more with different tubes. 

Cary Audio SLP 05 two chassis preamp sounds competitive to tha Mac at half the price.  a new one is on sale at cary direct for 5997.  love mine completely.  

Agree with @glennewick: swap in some NOS tubes to the C2500.   Telefunken in our C2500 was a major game changer.  Major.  If not, I would have sold that Mac a long time ago.  

Which Telefunken tubes? And, @elliottbnewcombjr I did not buy the Sonetto III. but the Olympica Nova III. I upgraded pre-purchase. I enjoy them.

I second the Aric Audio Motherlode suggestion. I had both the 2300 and the 2500 prior to the Motherlode...the difference in performance will make you question Mac and high end audio (read; exorbitant cost structure) in general.