how to make VTL2.5 sound warmer?

I have never owned your preamp. But you might try some isolation under it. Try anything. Try a book, some wood, anything. If that seems to work then you can try more expensive mainstream products that might do a better job. I did this type of experimentation with my BAT 50se. It took me 8 months to figure out that I should try isolation.

The result was that I finally understood the tube facination. I read a review where the 50se was described as being dry and solid state sounding. When I bought the unit and just sat it on my rack, that's exactly what I heard..dry and solid state sounding. Isolate that pre and I am hope you will here a difference as I did.

I own the VTL 5.5 with very similar tonal characteristics. I found it very sensitive to power cord changes and like the changes, particularly midrange, with use of a Cardas Golden Reference power cord.

Try some NOS tubes. I have a VTL IT85 (basically the VTL 2.5 as a pre), and replaced the 12AU7 with Amperex Bugle Boy 7316, and got more of everything: awesome palpable (rich, you can touch it!) midrange. Definitely worth it (I paid $175 for a matched pair, which is high, but I still would spend the same amount for the upgrade I got)