Which Mcintosh tube pre-amp?

The best is the new C2700. It also has a dac based on the ESS 9028 chip. This dac is an excellent one.

What about a used C2500 or C2600 tube preamp?

Up from the C-2600, the sound is a big upgrade.

So C-2600 or C-2700 , you’re in business. The latter has the newer dac ( da2 upgradeable module. )

The only difference between the C2500 and the C2600 is the newer DAC. 

Wrong stereo5, and I have proof below. 2 internal pics of the analog board side of the C2500 and C2600. Clearly different. ( so its not just the dac )

https://www.casimages.com/i/170104083930483890.jpg.html
https://www.casimages.com/i/17010408393284308.jpg.html

Techno_dude........................

looks like they shrunk the C2500 circuit board for the C2600.   The guys in the McIntosh group claim that sonicly,  the C2500 and 2600 sound the same except the dac.  

Look at it closely, it’s just not the same board, everything looks better made on the newer board. Shorter signal paths. Look also at the 2 rights tubes, they upgraded the input / output modules. The C2600 seems to be a nice upgrade in that regard.

The ESS 9018 dac module on the C2600 is Wadia inspired ( since they absorbed Wadia Digital) . Big upgrade compared to the older BB in the C2500.