I just installed some 3M damping sheets in my Belles 21 that I bought thru AMUSICDIRECT.COM $20 for a set of 4, very cost effective IMO and I can hear a difference. I mounted them inside the removable top of the pre. I also use Vibrapods as well.
Tube Preamp Isolation
Currently using Vibrapod cone/isolater combo under my Music Reference RM5 MKIII. I've got it on a Billy Bags rack. I've got an Audio Research LS15 on the way and am wondering what you guys use (if anything) under your tube preamp?