McIntosh Guy Going Tubes? (Maybe?)

I agree with ^
Keep the 601s and get the C2600.
I have the C2500 with an MC 452 and the MEN 220. Very happy with this combination 

I agree with tlong1958,
I have a C2500 with Genalex Gold Lion tubes and an MC452 and would not change.  I used to enjoy a C50 and when I received the C2500, there was a small, but noticeable change in the "musicality", especially voices and acoustic instruments; when I swapped out the stock tubes for the Genalex, those qualities improved even more.
The MC601's are such great amplifiers!  I personally would not change those out.

I put those same tubes in my C2500. Nice improvement 

I am in agreement with the others. I have a C2500 with the Gold Lion tubes in the line stage and a MC152 solid state amp=Heaven. 

If you want something a bit more musical or lush - Try 1 NOS RCA Long Black Plate 12AX7 from 1950’s in the input of the line stage on a C2500 or C2600. I agree on the Gold Lions for the phono (nice detail and probably as good as NOS telefunken ECC83) but I found the McIntosh stock 12AT7 on the output of the line stage to be the most musical - so I kept that (it is a Czech tube from JJ). I tried NOS Mullard but they are too dark for my taste.

The point is that with a bit of effort and not a huge expense you can tailor the sound to what works for you and your system. McIntosh may not be the last word in resolution but they know a thing or two about making an amp sound sublime.