C2600 recommendations

On my C2500, I am now using the Gold Lion tubes 12ax7.  Most of the guys on the McIntosh forum have done the same with their Mac tube preamps.  It gives you the best midrange of any of the 12ax7 tubes.

I agree with stereo5.  Replacing  the Mcintosh branded JJ tubes with Gold Lion 12AX7 (gold pins) is huge.  Also I've tried Mullard 12AX7's reissues (not the 4004's) and liked them a lot, cheaper too.
From what I've the read My C2500 is suppose to have firmware issues but not mine (fingers crossed).  Regarding other Mc pre's, They have made so many over the years it is hard to say.

@vegasears...……………...

The latest firmware addresses  the problem and fixes it.  Audio Classics updated my C2500 firmware before it was shipped to me.

I've enjoyed a C2500 now for two years, swapped out the stock tubes for Gold Lions, immediately noticed a difference, especially in the midrange.  I don't use the internal DAC.  I suspect the main sonic difference between the C2500 & C2600 would be the performance of the built in DAC, which I suspect is improved in the C2600.