Keep McIntosh C2500 pre-amp or upgrade to C12000?

If I was you I’d take a hard look at the new Spatial Audio Raven or Aric Audio Motherload preamps given what you’re looking for, and they cost less than half the price of the C12000. I think both offer a trial period, and I’d bet big $$$ you wouldn’t send either back and will be only too happy to sell your C2500 once you hear what you’ve been missing. Best of luck.

I just remembered your turntable discussion, what did you end up doing?

Are you still using the Sonus Faber Sonetto III speakers? They (and their ported design in your space) are making the most significant contribution to your system's sound characteristic. I would not expect any significant change in sound changing those preamp models.

You have very powerful 450 wpc solid state amp for those speakers 88 spl efficiency, and they have both a rear port and a downward reflected port at their base.

I would consider changing the speakers before changing the Preamp, you have 'tube sound' preamp already, I would make a plan for eventually tube amp which means more efficient speakers, and I encourage larger driver(s) no ports.

i will look into those, soix, thanks for the recommendation!

Yes, Elliott, still running all the original gear. The tube sound is what i was concerned making an upgrade but gaining no value in the sound. I am not dissatisfied at all just want to do make any upgrades worth the money spent. Thanks!

oh, on the turntable i went with a Pro-Ject Extension 9. Happy with it so far, but.....

You gotta get rid of the  ... but ..... I suspect you need nothing except time to enjoy what you have.